> One of the main reason why do I prefer the author signature (means the certificate) is as this allows an additional information within the signature (like company logo, the individual reason for approval, Acrobat version etc.).

These features also work for approval signatures.

Both author signatures (aka "certification signatures") and approval signatures in PDFs use X.509 certificates and digital signatures (to cryptographically secure the document with a connection to an identifiable person) and allow a visualization of the signature containing arbitrary information.

(Adobe really made things difficult by calling one of these two signature types "certification"...)

The difference is how the signature is displayed. Furthermore, up to the old ISO 32000-1 standard only the author signature could restrict which changes in general are allowed to the signed document. Nowadays, though, approval signatures can (further) restrict this.

Options for the author signature (certificate with visible signature)

Many thanks for explaining the difference between the author signature (with certificate) and approval signatures. This is much more clear now.

One of the main reason why do I prefer the author signature (means the certificate) is as this allows an additional information within the signature (like company logo, the individual reason for approval, Acrobat version etc.).

Please correct me if I am wrong, but from my experience, the "approval signature" does not offer that kind of functionality.

> Honestly, I do not understand the difference between "Digitally sign" and "Certify"

Technically they are very similar. "Certify" actually is a special case of a digital signatures that is reserved for the first signer of a document, also called "author signature". Thereafter a document can still be "Digitally signed" many times if the author originally allowed this. This signatures also are known as "approval signatures".

Whether you as initial signer create an author signature or an approval signature, is not relevant. The other signers must not use author signatures, though, they must use approval signatures.

> Another concern is that each signer can remove his certificate afterwards which is not optimal.

It's a choice of Adobe that each signer can remove their signature from a document. This strictly speaking is not according to the PDF specification.

Honestly, I do not understand the difference between "Digitally sign" and "Certify" (...no matter if with or without visible signatures).

I guess I have solved the issue now by doing following: I have placed my certification (with visible signatures) in 3 different fields by choosing "edit the form and add further certificate after signing". Then I removed all my certifications without deleting the fields where these were set. Thanks to this, the other 2 users are able to place their certified signatures. So this works well.

The only concern I have now is that each signer has to take care on the option when signing the file (means to allow others to sign as well). Another concern is that each signer can remove his certificate afterwards which is not optimal.

I'm not sure we correctly understand each other.

In the "Use a certificate" menu I see the options "Digitally sign", "Timestamp", "Validate all signature(s)", "Certify (visible signatures)", and "Certify (invisible signatures)".

When I asked "Have you tried using it to digitally sign instead?" I meant using the "Digitally sign" option there. That option also uses your personal, local certificate, it does not "require E-signatures through an Adobe Sign" as you say.

Thus, have you or the other prospective signers tried to use that option?

The users have tried to place their own certified signatures as well, but without success. Although I choose the option 2) that allows others to add their signatures, they are unable to proceed as the PDF file is "protected"

No, I did not, but this may be a great idea. I have 3 fields for 3 approvers, so let me try to see what happens when placing 3 times my own certified signature, then delete 2 of them so 2 signature fields would remain empty, and both users may place their signatures there

No, I did not as we prefer our own certified signatures. Good idea as well, but this would require E-signatures through an Adobe Sign. Works well for me as I do have Adobe Pro, but not for the initial requestor of the form who does not have Acrobat Pro and would need to purchase it.

>Unfortunately, the option 2) and 3) do not work properly, so when sending the PDF file that contains my digital signature and asking others to sign this document with their own certificates, the users are not able to proceed.

What do you mean by "the users are not able to proceed"? What exactly have they tried and how did it fail?

That being asked, have you prepared empty signature fields for those other users to use for signing?

Also you use your certificate to certify the document. Have you tried using it to digitally sign instead?

Screenshot for better understanding. I am using Acrobat Pro for MS Windows (2023.008.20555)