How to Draw a Line on a PDF: Straight or Freehand (Windows, Mac, iOS & Android)

One distinction worth knowing first: a line you draw here is an annotation — it sits on its own layer above the page, so you can move, restyle, or delete it without touching the text underneath. That is not the same as underlining selectable text (an underline follows the text) and not the same as a scaled measurement line (which reports a real-world distance). For markup, review, and pointing, the annotation line below is what you want, on every platform.

Part 1. Choose the right way to draw a line

Use the Line shape when alignment matters. Use Pencil when the path matters more than geometric precision. Neither is the same as underlining selectable text: an underline follows the text, while a line annotation can be placed anywhere on the page.

For the broader set of markup options beyond lines, see UPDF's complete guide to annotate PDF.

Part 2. How to draw a straight line on a PDF with UPDF (Windows, Mac, iOS & Android)

UPDF's Comment workspace provides a dedicated Line shape for a straight mark, and the same tool is available on desktop and mobile — so a line you place on your laptop stays intact when you reopen the file on your phone.

Open the document in UPDF — if you don't have it yet, download UPDF for free and install it in under a minute — then follow the steps for your device.

On Windows and Mac

Step 1. Open the PDF in UPDF and click Comment in the top toolbar.

Step 2. Open Shapes and choose Line. Choose Arrow instead only when the mark needs to show direction.

Step 3. Click at the line's starting point, drag to the ending point, and release. Hold Shift while dragging to lock the line horizontal, vertical, or to 45°.

Step 4. With the line selected, use the object controls shown in your version to change its appearance, move it, or delete it.

If the line needs to separate two rows, mark a diagram, or point to one exact sentence, zoom in before dragging — the extra page detail makes it easier to place both endpoints precisely.

On iOS and Android

The UPDF mobile apps carry the same Comment toolset, so you can draw with a finger or stylus.

Step 1. Open the PDF in the UPDF app and tap the Draw icon.

Step 2. Choose Line and set color, thickness, and opacity.

Step 3. Drag your finger or stylus from the start point to the end point. Tap a finished line to move, restyle, or delete it.

A stylus on an iPad or Android tablet makes this feel close to ruling a line by hand on paper — useful for separating rows on a form or connecting a margin note to a clause before a call.

Download UPDF for free to try the Line tool on a copy of your document — installation is free, and Pro features are available when you need watermark-free export or advanced tools.

Part 3. How to draw a freehand or curved line

UPDF's Pencil tool creates a freehand stroke rather than a straight shape. Choose Pencil when you need a curved or irregular path.

Step 1. Open the PDF and select Comment. On mobile, tap the Comment icon.

Step 2. Choose Pencil.

Step 3. Set the Pencil color, thickness, and opacity for the mark you want.

Step 4. Drag across the page (or with your finger/stylus on mobile) to create the curve or freehand line.

Step 5. If part of the stroke is wrong, choose Eraser and use partial or whole removal as appropriate.

Choose Pencil settings that keep the mark visible without covering nearby text. For a ruler-straight divider, switch to the Line shape instead of trying to steady the Pencil manually. For a broad highlight across text, use the dedicated PDF highlighting workflow rather than forcing Pencil to act like a highlighter.

Part 4. Draw a line online, or in Adobe — and when to use each

If you can't install software, a browser editor works on any device. If Adobe Acrobat is already on your machine, its commenting tools include a line option too.

Online (Smallpdf): open the Smallpdf editor, upload your PDF, open the shapes tool, and choose Line, or pick the pencil for freehand. Draw, then download.

Adobe Acrobat: open the PDF, choose All tools > Add comments > Draw Freehand > Line. Click at the line's starting point, drag to the ending point, and release. Acrobat Pro runs about $19.99/month (~$239.88/year) with no lifetime license, so it's mainly worth it for users already inside the Adobe ecosystem.

Local app or online — how to choose:

Download UPDF for free to draw straight and freehand lines on your own PDF — installation is free, and Pro features unlock when you need watermark-free export.

Part 5. Edge cases: when drawing or saving won't work

The line tool is greyed out, or nothing places. The PDF likely has a permissions password restricting annotation. You'll need the owner's authorization to unlock editing — only remove restrictions on files you have the right to edit. Request an editable copy rather than trying to bypass the restriction.

Dragging selects text or pans the page instead of drawing. The wrong tool is active. Return to Comment mode and choose Line (or Pencil for a curve).

The line won't select or move. If it was added as page content (drawn in an editor, or flattened on export) rather than as an annotation, it's baked into the page. Use the Edit tool to select it as a page object, or redraw it as an annotation.

The lines disappeared when opened elsewhere. Some viewers hide annotation layers by default, or the file was exported without comments. Re-export with annotations included, or flatten the lines into the page if they must show in every viewer (flattening makes them permanent).

Part 6. FAQ

Conclusion

To draw a line on a PDF cleanly, use the Line tool for a precise straight segment and Pencil for an irregular stroke — across Windows, Mac, iOS, and Android — then use the Eraser for unwanted Pencil marks. For scaled plans or diagrams, use a dedicated measurement workflow instead of a visual line. Keep a copy of the original before a large review pass, and test one straight line and one freehand mark to confirm the look you want.

For the full picture of highlighting, commenting, stamping, and drawing together, see our complete guide to annotate PDF.

Download UPDF for free to add straight and freehand line annotations to a copy of your PDF — installation is free, and Pro features are available when you need watermark-free export or advanced tools.