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Adding visible signatures to existing PDF files in Draw

Estimated read time: 3 minutes

Draw now has support for adding visible signatures to an existing PDF file. This is in contrast with the old functionality which was limited to invisible signatures.

First, thanks to the Dutch Ministry of Defense in cooperation with Nou&Off who made this work by Collabora possible.


The PDF format allows assigning a shape (a form xobject) to a digital signature in the PDF file, and if you use e.g. Adobe Acrobat, then it fills this shape with some visible information about the digital signature. Draw used to write a placeholder widget there (a 0x0-sized rectangle on the first page, at position 0x0). This is valid, but it’s not close to real-world signatures, where signing has a visual effect as well.

Results so far

Here is how this works in practice:

Figure 1. Demo of adding a visible signature to an existing PDF file in Draw

You can see how the 2 added signatures are visible and Adobe Acrobat confirms they are valid, too.

How is this implemented?

If you would like to know a bit more about how this works, continue reading… :-)

  • Signature lines were already working in Writer and Calc, this effort brings them to Draw, improving consistency.

  • Signing existing PDFs were already possible, this allows adding a visible signature with the correct markup. This is important for automated processing of PDFs, maybe even helps accessibility. (I think DocuSign doesn’t get this right currently.)

  • This uses the existing "export selected shape to PDF" code to produce that object, so it’s not a bitmap, but a scalable format. (As I know, DocuSign doesn’t do this, either.)

  • If you didn’t get the signature rectangle right for the first time, you can still move and resize it before the actual signing happens (Acrobat doesn’t support this currently, I believe.)

  • The generated object is locale-aware when it comes to the actual signature string and date format.

  • The feature works for multiple signatures and multiple pages as well.

  • The final step was this commit, with much more grounding before that one.

  • Note that the signing is a two step process: first you draw the signature rectangle and optionally finalize its position / size, and only then you use the Finish Signing button on the infobar to trigger the actual signing:
Figure 2. After drawing a signature rectangle, before finishing the signing.

If you use a HW-based certificate, this second step will ask for your certificate PIN.

Want to start using this?

You can get a snapshot / demo of Collabora Office and try it out yourself right now: try unstable snapshot. Collabora is a major contributor to LibreOffice and all of this work will be available in TDF’s next release too (7.1).

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