How to Sign a PDF on Your Phone Without an App
Your phone can already do this. Here is the fastest route on iPhone and Android, and the one case where a photo of your signature beats drawing it.
You do not need to install anything. Both iOS and Android can put a signature on a PDF using software that shipped with the phone, and the whole thing takes about a minute once you know where the button is.
On an iPhone
Open the PDF in Mail, Files, or Safari and tap it to bring it up full screen. Tap the Markup icon — a pen tip inside a circle — then the plus button in the bottom right, then Signature. iOS keeps signatures you have drawn before, so the second time you do this it is two taps. Draw with a finger, tap Done, then drag the signature box over the line and pinch a corner to size it. Share, then Save to Files, and the flattened copy is what you send back.
Markup lives in more places than people expect: Mail's reply view, Photos, and the Files app preview. If you cannot find it, the file is probably open in an app with its own viewer, like Gmail or Drive. Move it to Files first.
On an Android phone
There is no single answer here, because Android's PDF handling depends on the manufacturer. Samsung devices have Markup built into My Files. Pixel phones do not, and Drive's PDF viewer is read-only. The route that works on any Android phone is a browser-based editor: open the file, place the signature, download.
Draw or photograph
Drawing with a finger on glass produces a signature that looks like it was drawn with a finger on glass. For anything that will be read by someone who knows your handwriting — a lease, a loan document, a school form that goes into a file — photograph a real signature instead.
Sign a blank sheet of white paper with a black pen, using thicker strokes than feel natural, and photograph it straight on in daylight. A good editor knocks the white background out so what lands on the page is the ink alone. The result is markedly better than anything you will draw on a screen, and you only have to do it once.
When it is not enough
A signature you draw or paste is an image of a signature. For most documents that is exactly what is being asked for, and it carries the same weight as signing on paper and scanning the page.
Some documents want something else. If a form says it requires a digital certificate, a qualified electronic signature, or signing through a named platform, an image will be rejected — not because it looks wrong, but because the verification is cryptographic and an image has nothing to verify against. Notarised documents have their own requirements, which vary by state and country. When a form is specific about the method, take it literally and check with whoever issued it.
Getting it back out
Whatever you use, the file you send has to be a PDF with the signature baked into the page, not one with a separate annotation layer that some readers display and others quietly ignore. On iOS, using Share rather than simply closing the Markup view is what does this. In a browser editor, it is whatever the download button produces.
actuallyfreepdfeditor.com runs in your phone's browser, so it behaves the same on a Pixel as on an iPhone. You can draw a signature, type one, or upload the photo of the one you signed on paper, then drag it onto the line and download the finished file. The PDF is opened and rewritten on your own device rather than sent anywhere.
One habit worth keeping: after you download the signed copy, open it again and look at it. A signature placed slightly off the line, or sized so it overlaps printed text, is the sort of thing that gets a form sent back. Ten seconds of checking saves the round trip.