Applications may allow users to modify PDF documents. Users should not have to wait for the entire file—which can contain hundreds of pages or more—to be rewritten each time modifications to the document are saved. PDF allows modifi- cations to be appended to a file, leaving the original data intact. The addendum appended when a file is incrementally updated contains only those objects that were actually added or modified, and includes an update to the cross-reference table. Incremental update allows an application to save modifications to a PDF document in an amount of time proportional to the size of the modification rath- er than the size of the file.
In addition, because the original contents of the document are still present in the file, it is possible to undo saved changes by deleting one or more addenda. The ability to recover the exact contents of an original document is critical when digi- tal signatures have been applied and subsequently need to be verified.