PDF is considered the standard format
Why Word and Excel documents are not the first choice for digital signatures?
The use of Word and Excel files does not provide reliable protection against subsequent changes to the documents. Thus, it cannot be safely ruled out that documents are manipulated or changed after they have been signed. Experts refer to this as document integrity, which is not guaranteed.
This is why the PDF format has become the standard format for the secure signing of documents. PDF/A in particular is used by default as a manufacturer-independent long-term format. It can be assumed that this will still be representable one-to-one on various operating systems and with various applications decades from now. Many archiving systems and software programs for checking signatures in PDF documents (such as Adobe Reader) are available.