A contract arrives by email while you are away from your desk. It needs your signature by the end of the day. You have your phone. You do not have a printer, a scanner, or a stylus. You do not want to install a PDF signing app for a one-time task. Can you sign that document using only the tools built into your phone?
Yes, both iPhone and Android include built-in markup and scanning tools that can capture your signature and place it on a PDF. The process takes under two minutes and requires no downloads, no accounts, and no app installations. The Sign PDF workflow on mobile uses the camera and touchscreen together to produce a clean, legally presentable signature.
Signing a PDF on iPhone Using Markup and Camera
Open the PDF in the Files app. If the PDF arrived by email, tap the attachment and choose Save to Files. Navigate to where you saved it in Files and tap to open. The PDF opens in the built-in viewer. Tap the Markup icon, which looks like a pen tip in a circle, in the top-right corner. The Markup toolbar appears at the bottom.
Tap the plus button in the toolbar and select Signature. If you have never created a signature, tap Add or Remove Signature, then tap the plus icon to create one. You have two options. The first is to use your finger to draw your signature on the touchscreen. Take your time. A slow, deliberate stroke produces better results than a rushed scribble. The second option, and this is the one that produces the most professional-looking result, is to sign a piece of white paper with a dark pen, hold it up to the camera, and let the iPhone capture it. The camera method produces a signature that looks almost identical to a wet-ink signature because it is a photograph of an actual pen signature.
After capturing the signature, it appears as a movable object on the PDF page. Drag it to the signature line. Pinch to resize it. Tap Done when finished. The signed PDF saves automatically in place. Share it via email, Messages, or any app directly from Files. The entire process, from opening the email to sending back the signed document, takes under two minutes.
Signing a PDF on Android Using Built-In Tools
Android does not have a single universal PDF signing tool pre-installed on every device, but Google Drive and Google Files, both available on virtually every Android phone, provide the capability. Open Google Drive, tap the plus button, select Scan, and capture a photo of your signature on paper. Save the scanned image. Then open the PDF in Google Drive's viewer or upload it to The tool's sign tool through your phone's browser. Use the Add Signature function to place the scanned signature image onto the PDF's signature line.
Alternatively, use your phone's browser to open WukongPDF's sign tool directly. Upload the PDF, select Add Signature, and choose to capture your signature with the camera or draw it on the screen. Place the signature on the document, adjust the size, and download the signed PDF. The browser-based approach works identically on iPhone and Android and requires no app installation.
The Camera Method: Capturing a Real Pen Signature
The camera method produces the best-looking result for formal documents. Use a white piece of paper and a dark blue or black pen. Sign larger than you normally would. A signature that is 2 to 3 inches wide on paper captures more detail and scales down cleanly. Place the paper on a flat surface with even lighting. Avoid shadows across the signature. Hold the phone directly above the paper, parallel to it. Take the photo, then crop it tightly to the signature area so only the white background and the dark ink remain.
After importing the signature photo, most signing tools automatically remove the white background, leaving only the ink on a transparent background. This transparent signature overlays cleanly on any document without a white rectangle around it. If the tool does not remove the background automatically, you can adjust the contrast or use the tool's built-in background removal feature. The tool's sign tool includes automatic background removal for signature photos.
When the Built-In Approach Is Enough vs When You Need More
The built-in phone tools cover the most common signing scenario: one person signing one document in one place. For contracts requiring multiple signatures from different parties, or documents needing a cryptographically verifiable digital signature rather than just a visual one, the built-in tools fall short. They produce a visual signature image on the page, not a digital certificate that proves the document has not been altered after signing.
For everyday documents, permission slips, vendor agreements, internal approvals, lease renewals, the visual signature produced by the phone camera method is perfectly adequate and legally acceptable in most jurisdictions. For high-stakes documents, real estate contracts, notarized forms, regulated financial agreements, a dedicated e-signature platform that provides an audit trail, identity verification, and tamper-evident sealing is the appropriate choice. The tool's PDF Tools cover the visual signing needs for everyday documents, while providing the option to add digital signatures for documents that require cryptographic verification.
After signing, flatten the PDF to lock the signature in place. Flattening converts the signature from a movable annotation into permanent page content. Without flattening, the recipient can accidentally move or delete the signature by tapping it in their PDF viewer. The tool's flattening option is available after placing the signature. Apply it before sending the signed document to ensure the signature stays exactly where you placed it.
One practical tip for the camera method: sign on unlined white paper with a pen that produces a consistent, medium-weight line. Gel pens and rollerball pens work better than ballpoint pens because they produce a darker, more uniform ink flow that photographs cleanly. Avoid pencils, which produce faint gray lines that the camera struggles to capture. Avoid very fine pens, which produce lines too thin to survive the background removal process. After photographing, most tools let you adjust the contrast threshold. Slide it until the signature appears dark and crisp against a clean transparent background. Too much contrast will make the signature look jagged. Too little will leave a gray haze around the letters. The sweet spot produces a signature that looks like it was created digitally rather than photographed from paper.
For documents requiring initials on every page, not just a signature on the last page, the built-in phone tools become tedious because you must place the initials manually on each page. A faster workflow for multi-page signing: sign and initial the first instance, then use a PDF editor to duplicate the signature annotation across all required pages. WukongPDF supports adding a signature once and applying it to multiple pages by specifying the page range. This batch placement feature turns a 20-page initialing task from 20 manual placements into one configuration and one click.