Extract signature from signed PDF and get all information related to that signature and validate signature is authenticated or not, authentication means signature is attached in PDF is generally generated by or based on a certificate or key so when we need to check that signature is generated by a specific key or certificate then we will able to do that.

Please refer this stackoverflow solution for this functionality in which they have used pdfreader library for that:
https://stackoverflow.com/questions/72556172/how-to-locate-digital-signatures-in-pdf-files-with-python