How to sign a PDF on your phone
Last updated: 2026-07-27
A form arrives by email, it needs your signature, it needs to go back today, and your laptop is somewhere else. You do not need a printer or a scanner. A phone can capture a signature, place it on the page and send back a new PDF in minutes — provided you know the two places the process stalls: making a signature image that does not look like a photocopy of a shadow, and getting the attachment out of the mail app and into a browser.
Step one: get a signature image
Option A — sign on paper and photograph it
The advantage is that it is genuinely your handwriting, at full speed, with the loops you actually make. Use a plain white sheet and a black felt-tip or gel pen, and sign larger and heavier than you normally would. A standard ballpoint photographs as a thin grey scratch that all but disappears once the image is scaled down to fit a signature line.
Option B — draw it on screen
Any notes or drawing app with a black brush on a white canvas works, and the result has no lighting problems at all. The catch is fingertip control: finger-drawn signatures come out blunt and slow-looking, and often do not resemble your real one. With a stylus or an Apple Pencil the result is excellent. If the app can export with a transparent background, do that — it saves you a step later.
Why photographed signatures come out dirty
If background removal leaves a grey smear on one side of your signature, the cause is almost always uneven lighting. When you hold a phone over a sheet of paper, your head, shoulder and arm block part of the light. Your eye compensates and still reads the paper as white, but the pixel values do not: the lit side sits around 250 while the shaded corner drops to 210 or 220. Background removal makes white and near-white pixels transparent, so the bright region clears perfectly and the shaded region survives as a visible grey block stuck to one edge of your signature.
Fix it at capture time, not afterwards:
- Shoot in daylight near a window. A single ceiling bulb at night is the worst possible setup.
- Move the paper so light reaches the whole sheet evenly, rather than moving yourself.
- Hold the phone slightly off to one side so your own shadow does not fall across the page — while still keeping the paper looking rectangular in frame, not trapezoidal.
- Turn the flash off. It blows out the centre and darkens the corners, making the imbalance worse.
- Crop tightly around the signature afterwards. Less paper in frame means less shaded paper to deal with.
Better still, use the document scanner built into your phone: Scan Documents in iOS Notes, or Google Drive on Android. These correct perspective and flatten brightness across the page, giving a far more uniform background than a plain photo. If the scanner only outputs PDF, look for an option to save as an image.
Getting the PDF out of your email
This is where most people get stuck. Tapping an attachment to preview it does not make the file available to your browser — you have to save it to storage first.
iOS: open the attachment, tap the share icon, and choose Save to Files. On My iPhone or iCloud Drive both work. Then open StickPDF in Safari or Chrome, tap the open button, and pick the file from the Files browser that appears.
Android: in Gmail, tap the download arrow on the attachment. It lands in your Download folder. In the browser, tap the open button and choose it from Recent or from Downloads in the file picker.
Placing the signature
Pages render at screen width. Scroll to the page that needs signing and use that page's + Add image button rather than trying to drop the file — on a small screen it is easy to be looking at page 4 while your finger is technically over page 3. Drag the signature onto the line, then drag the corner handle to size it. The aspect ratio stays locked, so it will not stretch.
If the white background hides the printed signature line, tap Remove background. White and near-white pixels become transparent and the signature sits on the line properly. Restore background undoes it if the result looks patchy — and if it does look patchy, re-shooting in better light beats fighting the image.
Then download. On iOS the new PDF goes to the Downloads folder inside the Files app; on Android, to Download. Reply to the original email, tap attach, and pick it from there.
One thing to be clear about
This places a picture of a signature on a page. It is not a cryptographic digital signature backed by a certificate, and it carries no built-in proof of who applied it or when. Whether a signature is acceptable depends on the document, the counterparty and the law where you are, so for anything consequential — property, employment, court filings — check what form of signature the recipient actually requires before sending. Also worth knowing: everything here happens in your browser. Neither the PDF nor your signature image is uploaded anywhere.