Sign PDF
Draw a signature, type one, or photograph the one you already have on paper. Put it where you want it and save, with the document you started from still there, byte for byte, underneath.
- The signature is appended, not rebuilt: your original bytes are the first bytes of the signed file
- Text stays text: nothing is flattened into a picture
- Draw with a pen that thins as you move, type in five hands, or lift one off a photo
- Place as many as you like, on any page, and drag them where they belong
Nothing is uploaded. The file is read and written in this tab.
Click the page to place the signature. Drag to move, pull the corner to resize, arrow keys to nudge.
- Original bytes
- —
- Text
- —
- Added
- —
What gets added to your file
| Object | What it is | How many |
|---|---|---|
| Image | The signature itself, as a picture | One, shared by every placement |
| Bracket streams | A q and a Q around the page's own content, so its graphics state cannot leak into ours | Two per signed page |
| Content stream | The instructions that draw the signature at the right size and place | One per signed page |
| Page dictionary | A replacement pointing at the new content and resources | One per signed page |
| Cross-reference | A new section whose /Prev points at the old one | One, at the very end |
None of that disturbs what was already there. The original objects stay at the byte offsets they always had; the new cross-reference simply points somewhere else for the pages that changed.
Appended, not rebuilt
Almost every tool that puts a signature on a PDF does it by loading the document, modifying it in memory, and writing a new file. The result is usually correct. It is also a completely different set of bytes: every object renumbered, every stream re-compressed, the structure rearranged by whatever library did the work.
That matters if anyone later asks whether the document you signed is the document you
were sent. After a rewrite there is no way to tell. After an append there is: the original
file is the first N bytes of the signed one, and cmp will say so in a second.
This page refuses to save if that check fails, which is not a slogan: it is an assertion in the code, and the byte comparison runs before the download starts.
What a drawn signature needs to look right
A line of constant thickness does not look like handwriting. Real ink is laid down faster on the long sweeps and slower on the turns, so the stroke thins where the pen moved quickly. The pad measures the speed between each pair of points and varies the width accordingly, which is most of the difference between a signature and a wire.
Undo takes back one stroke rather than the lot. Most attempts go wrong on the last flourish, and clearing the whole thing to fix that is how people give up.
Lifting a signature off paper
The upload tab takes a photo or a scan and removes the paper by brightness: anything lighter than the threshold becomes transparent, and what is left is re-coloured to the ink you chose, keeping its own darkness so a pencil stroke stays soft.
It is crude and it works, because the ink is the dark part. The threshold is a slider rather than a constant because a phone photo in a dim room and a flatbed scan are nothing like each other.
This is not a digital signature
It is worth being plain about. What this does is draw a picture of your signature onto the page, an electronic signature. It is the same thing as signing a printed copy and scanning it back, and in most jurisdictions it carries the same weight for ordinary agreements.
What it is not is a digital signature: a cryptographic seal, backed by a certificate, that proves who signed and shows whether a single byte has changed since. That needs a certificate authority and an identity check, and no web page can do it for you.
If someone has asked for a "digitally signed" or "certificate-based" PDF, this is not that. If they asked you to sign a form and send it back, it is exactly that.
Questions
Is my document uploaded?
No. It is read, drawn on and written in this tab. You can watch the network panel stay empty while you use it.
Is this legally binding?
An electronic signature is accepted for most ordinary agreements in most places, including under eIDAS in the EU and the ESIGN Act in the US. It is not a certificate-based digital signature, and where one of those is specifically required this will not substitute for it.
Does the document stay searchable?
Yes. Nothing is flattened. The text is the same text objects it always was, only a picture has been drawn on top.
How do I check the original was not altered?
cmp -n $(stat -f%z original.pdf) original.pdf signed.pdf on macOS, or
head -c $(stat -c%s original.pdf) signed.pdf | cmp - original.pdf on Linux.
Silence means every original byte is intact.
Can I sign more than one page?
Yes, as many as you like, and more than once per page. A dot on the page row marks every page that already carries one.
Why was my PDF refused?
Either it is password protected, or its cross-reference table is damaged past the point where a new section can be chained onto it. Rewriting such a file would be possible, but not without breaking the promise this page is built on.