Flattening a digitally signed document would not make a lot of sense if you need any digital signature features beyond an appearance. If all you need is an appearance, then use a stamp instead. If you do need a digital signature, you can set it up so that the document is locked after the signature is applied, preventing further changes.
Thanks for your response. Before I started looking into the 'flatten' option, we tested sending a doucment with the digital signature to a another person. When that user opens the document they are prompted with 'there is a problem with this signature' or 'this signature needs validating'. We don't want everyone who looks at a document to get prompted like this. Is there a way to get around these messages? Maybe I just haven't set the security up properly, electronic signatures are new for us.
Validating signatures is the normal and most important thing about them. That is how they work. This is fundamental stuff which you must explain to the users of the document as surely as they need to know how to use a pen for paper signatures.
Consider: if you come upon a PDF that is signed by your boss, how do you know it is signed by your boss? Because the signature has his name on it? Hardly the proof needed to fire a man or release a million dollars.
But if your boss sent his public key to you before, you can check the signature in the file against the public key, and this is absolute proof. Validating has to be done every single time, otherwise it is like sending a paper document with "trust me, this one's fine" written on a sticky note.
However, a group of people can share signatures and arrange that validation is automatic when files are opened. Don't know the details.
It depends. It's possible to set up the client machines so that digital signatures are automatically verified (e.g., Edit > Preferences > Security > Digital Signatures > Verify signatures when the document is opened). For self-signed digital ID that you create in Acrobat, each user would have to add your certificate to their list of trusted certificates for this to work.
Also, note that you're dealing with digital signatures, not electronic signatures. They have different meanings, although in some places in Europe both terms are used to refer to digital signatures. Understanding the differences may help as you investigate more.