It should be said that PDF signatures are a very fragile design, leading to a pletora of security issues: https://pdf-insecurity.org/

A core problem is that a pdf signature does not necessarily cover a complete file, but can be a partial signature. This adds a whole lot of complexity and unclarity around what is actually signed, allowing all kinds of attacks. I feel this is all so problematic that if you want to sign PDFs it's probably better to not use PDF signatures, but some form of outside signatures over the whole file.