How can I validate a pdf's digital signature with evince?

A while ago I received a PDF file that was digitally signed.

The Document Viewer (Evince) shows me that the digital signature was not validated.

How do I add the file's digital signature to evince so that the warning disappears?

Unfortunately, you can't (now on Gitlab) do this at the minute with the standard version of evince, as a developer explained on the official mailing lists:

This feature isn't implemented in Evince, mainly because isn't supported by poppler, the underlying library for PDF support.

This is indeed the main point; you would need to patch poppler with the digital signature support as well as evince.

However, there is a interesting project in progress to provide just the functionality you want; the details are explained on this github page entitled 'evince signatures', where all the patches are available.

As well as on the github page, there is more information on the project on this site; it is clear they hope to integrate it upstream in the future. The developers note that

the most used software for reading pdf documents on Windows (Acrobat Reader) has support for showing and validate signatures in pdf documents. On modern Linux desktops that doesn't yet exist and that's the purpose of this project, change Evince document viewer (default gnome pdf reader) to add those features.

So, you can experiment with the github development code if you wish, but it may not be a good idea to patch poppler as other programs depend on it; however, it is likely that the digital signatures feature will be available soon if it is integrated upstream. Judging by the evince screenshots for a valid and invalid signature the patches do seem to be enabling just the features you are interested in.