PDF digital signature is it for page or whole document?

Is the PDF digital signature f开发者_如何学编程or a page or for a whole document? Can a page be signed ?

Although the signature is usually placed on a particular page of the document, the entire document is actually signed. However, PDF format supports so-called 'incremental updating' technique that allows to add pages to the document without invalidating the existing signatures (and e.g. Adobe Reader has the capability of showing the piece of document covered by the signature, even if it was updated after the signature has been created).

The digital signatures are based on the physical layout of the bytes in the PDF file. The signature object defines the ranges of bytes (start offset and length) in the PDF file that are used for computing/validating the signature. Technically you can define a set of ranges that cover only the objects that compose a page, but any application that validates the signature ignores the logical payload of the bytes included in the signature. If you write your own PDF signer and validator you can implement the logic that signs a single page. All the tools that sign PDF files will always sign the document, not a specific page in a document.