Apple Made iPhone Duo. UPDF Already Had PDF Duo

iPhone Duo: $1999. UPDF for PDF Duo: free (free download; comparing, splitting, and dual-view are free; AI features like translation run on UPDF AI credits).

Apple named a phone Duo. Cute. I've been running Duo since before it could fold. No hinge, no crease, no dramatic bend — just two things on one screen, which is the entire point of a "duo" in the first place.

Because here's what nobody mentions between the keynote applause: you don't need a folding phone to look at two documents at once. When you're checking a revised contract against the old one, reading a paper while you ask questions about it, or navigating a long report while keeping the current page in sight, you just need a workspace that puts two views in front of you — and UPDF has done that on Windows, Mac, iPhone, iPad, and Android this whole time.

That's what I'm calling PDF Duo: two views, no switching. "Duo" here isn't one fixed layout — it's whatever puts two things in front of you at once: two files compared side by side, one file next to an AI panel, a document split into two panes, an original beside its translation, or the same file handed from your phone to your Mac. Here's every way to get it, no folding required.

Part 1. Which "Duo" do you need?

Before the steps, here's the fast way to pick the right one for your task and device.

What you want	Use this	Where
Spot the differences between two versions of a file	Compare Files	Windows, Mac
Read a document while asking AI about it	Read view + AI panel	Windows, Mac
Two separate files open side by side	System split-screen / Split View / Multi-Window	Windows, Mac, iPad, Android
Thumbnails next to the page you're reading	Widen the Thumbnails panel	Windows, Mac
An original next to its translation	Bilingual layout / Side-by-Side Translation	Desktop & mobile app / Online
The same file across two devices	UPDF Cloud sync / Handoff	Cross-device

Part 2. Method 1: Compare two PDFs side by side (the real "PDF Duo")

The Compare Files feature puts both PDFs in one window, side by side, and pulls every difference into a panel on the right. It's available in the Windows and Mac desktop versions.

1. Launch UPDF and go to Tools > Compare PDFs from the home page.
2. In the window that opens, add the two files with Select File — you can pick from Recents (recently opened in UPDF) or from local storage.
3. Optionally tune the comparison: ignore text attributes like font size and color, ignore spaces, or use the filter to include only Text, Image, Path, Shading, or Page.
4. Choose a page range and click Compare.

The right panel lists every change — text edits, image and path changes, inserted or deleted pages — so you can see what's different at a glance. Turn on Highlight Result to color the changes on the pages themselves, and Sync Scrolling to move through both files line by line at the same time. When you're done, use Export to save the comparison and share it with your team.

Part 3. Method 2: Read on one side, ask AI on the other

One tool, two working states at once. In the UPDF desktop app you keep the PDF open in the reading view on one side and open the AI panel on the other, so the document you're reading and the answers about it sit next to each other — no switching windows, no losing your place. Ask for a summary, an explanation of a dense paragraph, or a quick answer, and the source stays right there for you to verify against.

Also read: How to AI Chat with PDF Files: The Ultimate Document Interaction Guide

The split isn't fixed, either: drag the edge of the AI panel inward or outward to rebalance the two sides — give the document more room when you're reading, or widen the panel when you're deep in a back-and-forth with AI.

Part 4. Method 3: Two PDF windows via system split-screen

1. Open the first PDF in UPDF.
2. Open the second PDF, then pull it out into its own window so both are visible at once instead of stacked as tabs. Two ways to do it: right-click the tab and choose Move Tab to New Window, or just drag the tab out of the tab bar and drop it on the desktop.
3. Snap the two windows side by side:
   • Windows: drag one window to the left edge until it snaps to half the screen, then pick the second window for the other half (or select a window and press Windows key + Left/Right arrow).
   • Mac: hover over the green button at the top-left of a window, choose Tile Window to Left/Right of Screen, then pick the second window for the other side.
4. Drag the middle divider to resize each half.

Part 5. Method 4: Split-screen on iPad, and Android

iPad — Dual Screen Mode

UPDF for iPad puts two UPDF windows side by side so it's easier to move files between them.

• Tap the "…" on a file and choose Open in New Window. The screen splits automatically; drag the divider to resize, or drag it to an edge to exit.

• Or tap the tab at the top, use the Multitasking button ("…"), choose Split View, and tap UPDF again for the other half.

• You can also enable Windowed Apps in Settings → Multitasking & Gestures, then drag a second app to the center of the screen, open the second app from your iPad Home Screens, or in App Library

Android

UPDF for Android supports split-screen — pair it with another app, or open two PDFs:

• Step 1. Long-press a PDF (or tap its "…" icon) and choose Open in Multi Window.
• Step 2. Swipe up from the bottom of your phone to bring up your phone's multitasking vie and tap the UPDF Home page.

• Step 3. Open a second document, swipe up again, and tap the Window icon to send one document into split-screen (steps vary by device).
• Step 4. Adjust the position of the two windows.

Part 6. Method 5: Widen the Thumbnails panel into a dual view

The Thumbnails panel isn't stuck at a narrow width. Drag its edge outward until the thumbnails sit at about the same size as your reading view — effectively a two-panel layout inside one window, with your page overview on one side and your reading page on the other.

1. Open your PDF and click the Thumbnails icon in the left sidebar.
2. Grab the right edge of the panel and drag it outward.
3. Keep dragging until the thumbnail pane and the reading pane are balanced side by side.

This is handy for image-heavy or design-heavy PDFs where the tiny default thumbnails are hard to read.

Part 7. Method 6: A PDF next to its translation

Sometimes the "duo" you want is one document in two languages. UPDF handles this two ways, and the right one depends on whether you're in the app or the browser. Unlike simply viewing two PDFs at once, translation runs on UPDF AI — the free plan includes a set number of AI uses, and heavier use is covered by a paid AI plan.

Bilingual layout (desktop and mobile app)

When you translate a PDF in the app, choose the output style. Instead of a clean, full-page replacement in the target language, pick the Bilingual layout to get the original and the translation side by side for line-by-line verification. Because it's generated in the app, you can download the result as a formatted PDF with the original layout preserved.

How the Translated Version Looks Like

In the UPDF mobile app, go to the UPDF AI panel and switch to Ask PDF mode. There you'll see the PDF Translation feature — it also lets you translate documents while keeping a bilingual side-by-side layout.

Side-by-Side Translation (UPDF AI Online)

In the browser, UPDF AI Online has a dedicated Side-by-Side Translation interface that shows your original file and the translation at the same time. This is the browser-based way to view a source and its translation together without installing anything — useful on a locked or restricted machine.

In addition to the methods above, UPDF provides several other options for translating PDFs. For more details, please refer to the video tutorial below.

Part 8. Method 7: The same file across two devices

The last duo isn't two views on one screen — it's one file across two devices, so your work follows you.

UPDF Cloud sync

UPDF works across Windows, Mac, iOS, and Android, and one account carries your files between them. Save a file to UPDF Cloud on one device and pick it up on another without emailing it to yourself.

Handoff (iOS ⇄ Mac)

With AirDrop and Handoff enabled on both your iOS device and your Mac, open a file in the UPDF mobile app, then launch UPDF on your Mac — the file appears in the Recents list, ready to keep working on the bigger screen.

Part 9. Compare vs. split-screen vs. dual view vs. bilingual

These sound similar but solve different problems. Think of it like a desk:

• Compare Files is putting two versions of the same document on the desk and having someone circle every difference for you. Use it to find what changed.
• Split-screen is putting two separate files on the desk, side by side, to read at the same time. Use it to cross-reference.
• Dual view (widened Thumbnails) is laying one file open with its table of contents spread out beside the page. Use it to navigate a long PDF.
• Bilingual layout / Side-by-Side Translation is the same content in two languages on the desk together. Use it to verify a translation.

Setup	Files involved	Best use	Where
Compare Files	Two versions of one document	Find differences	Windows, Mac
Read + AI panel	One file + AI	Study / review with answers alongside	Windows, Mac
System split-screen	Two separate files	Cross-reference	Windows, Mac, iPad, iPhone, Android
Widened Thumbnails	One file	Navigate a long document	Windows, Mac
Bilingual layout	One file, two languages	Verify a translation offline	Desktop & mobile app
Side-by-Side Translation	One file, two languages	Verify a translation in-browser	UPDF AI Online
Cloud sync / Handoff	One file, two devices	Continue work elsewhere	Cross-device

Part 10. Edge cases

• "Compare isn't on my iPad." Compare Files is a desktop feature (Windows and Mac). On mobile, open the two files with split-screen instead, or run the comparison on your desktop and sync the result.
• "I opened two files but they're stacked as tabs." On desktop, tabs live inside one window. Open the second file in its own window, then use system split-screen to place them side by side.
• "My iPad won't split." iPad split behavior depends on iPadOS multitasking. If Split View won't trigger, check Settings → Multitasking & Gestures, then retry from the file's "…" menu.
• "The file didn't show up on my Mac via Handoff." Make sure AirDrop and Handoff are enabled on both the iOS device and the Mac, and that you're signed in to the same account; then open the file on mobile first and launch UPDF on the Mac to see it in Recents.
• "My translation lost its formatting." Use the app's Bilingual layout (or a translated file downloaded from the app), which preserves the original layout — rather than copying translated text out of a browser session, which carries none.

Part 11. FAQs

The takeaway

"Duo" doesn't need a folding screen — it needs two views and one workspace. Compare two contract versions on a laptop, read a paper while AI answers beside it, cross-reference two files on a tablet, check a translation against its source, or hand a file from your phone to your Mac. UPDF puts both halves of the job in front of you on whatever device you're already holding. Apple can keep the hinge — I'll keep the two views. That's PDF Duo: no switching.