A short overview about the process of signing a PDF document with single or serial signatures is given. The special features of PDF to support signature workflows like certificate signatures and allowing for dedicated modifications after signing is presented. This is the origing of some of the complexity of the task to validate a signed PDF. We show which public standards on signature validation are available and how they affect the validation process. During the presentation various real-world scenarios of validating signed PDFs are demonstrated. Another topic will be the interpretation of validation reports in order to better understand the result if you don't receive a "green" checkmark.