The usual way to sign a PDF that someone sent you is to print it, sign the paper, and scan it back, which takes a printer and a scanner for a single line of ink. The other common route, drawing your name with a trackpad, gives a signature that doesn't look like yours. A third option is a photo: sign a sheet of white paper once, take a picture, and let the PDF editor keep only the ink. The steps below do that in the PDF Editor of the osFoundry desktop app, on an example freelance agreement.
Photograph the signature once
Sign a plain white sheet with the pen you normally use and take a photo of it in bright, even light. The editor's own hint says the same: use the flash if the room is dim. The photo only has to be readable; it doesn't have to be straight or cropped, because the next step finds the signature itself.
Turn the photo into a signature
Open the PDF, then choose All tools, Fill & Sign and Add signature. The Create signature dialog has four tabs; pick Scan, click Choose photo and select the picture.
The editor draws a frame around the signature and shows the result underneath: just the ink, on a see-through background, with the paper gone. It keeps what is darker than the paper, which is why even light matters. If the preview looks thin or has paper specks in it, the Ink sensitivity slider (it starts at 50) adjusts how dark something has to be to count as ink. Click Apply, and the signature is saved under Sign yourself in the Fill & Sign panel, ready for every later document.
Place it on the line
Click the signature line in the document. The signature lands there at a sensible size, and a drag moves it if it sits a little high or low.
At this point the signature is a sticker on top of the page. It is stored as a stamp annotation, the same kind of object as a comment or a highlight, so another PDF reader can select it, drag it somewhere else or delete it. That is fine while you are still deciding where it goes, and not what you want in the file you send.
Flatten it into the page
Open the command search at the top (Find tools and commands, or Cmd+K), type Flatten and choose Flatten comments.
Flattening turns annotations into part of the page. The signature is no longer an object sitting above the document but drawn in the page content, like the printed text. Save the file with the save button in the toolbar, and it is ready to send back.
On the example file, reading the saved PDF back with PyMuPDF showed 0 annotations and one image on the page, a 1032 × 198 px ink picture with its own transparency, where the version before flattening held one stamp annotation on top of the page.
A signature drawn this way is a picture of a signature, which is what most freelance and business PDFs ask for. It is not a cryptographic digital signature, and a contract that requires one needs a different tool.
The desktop app is free for macOS, Windows and Linux from the osFoundry download page, and everything above runs on the computer without uploading the PDF anywhere, as described under local-first privacy.
