Can I modify a document after it is digitally signed?

Yes. If a document was modified after it was signed, the property SignaturesInfo.documentModifiedSinceSignature will be true, indicating that the digital signature does not cover the entire document. Typically, if a document is modified after being digitally signed, the digital signature would be considered invalid. However, in the case of PDFs, you can still modify the document in certain ways without invalidating the digital signature. By default, certain actions such as adding long-term validation (LTV), a timestamp, filling forms, or adding another digital signature are allowed if the document is saved incrementally (see Enable Incremental Saving of PDFs Using JavaScript). Any other type of modification to the document would result in the signature being recognized as invalid in various third-party PDF viewers, such as Adobe Acrobat or Reader.