Digital signature

These are Apple's instructions on how to do that:

Fill out and sign a PDF form in Preview on Mac - Apple Support

If you talk about a digital signature you mean signing a PDF document using a X509 key, right?

I have been looking of this feature for a long time on apple. Adding a graphic scan of your handwriting as you can do with preview is not digitally signing a document.

Here is an explanation by Adobe what a digital signature in a pdf is:

https://www.adobe.com/devnet-docs/acrobatetk/tools/DigSig/Acrobat_DigitalSignatures_in_PDF.pdf

LibreOffice has the feature to digitally sign existing PDF files:

https://help.libreoffice.org/6.1/he/text/shared/01/signexistingpdf.html

The more difficult part is to get a certificate that can be verified by others.

Similar to SSL certificates there are many paid services.

A free option for people can be CAcert, It is a trust system where you must identify yourself to others.

http://www.cacert.org/

I happen to have a digital certificate issued by my bank for signing purposes. With a few modification I was able to convert the certificate format following these instructions:

https://hamonikr.org/board_tipntech/49551

Eventually Libreoffice needs a KeyManagement tool. The one that LibreOffice can use is the certificate management Integrated in firefox:

https://www-archive.mozilla.org/projects/security/pki/psm/help_21/using_certs_help

I wish the certificate management and signing capabilities were more integrated in the system and that preview would at least offer the possibility to verify and validate the signature.

One free tool is Adobe PDF reader:

https://helpx.adobe.com/acrobat/using/validating-digital-signatures.html

There are other solutions avaliable one could be:

https://pyhanko.readthedocs.io/en/latest/cli-guide/signing.html

[Edited by Moderator]