I have a document that is over 1200pgs, and is made up of several other combined PDFs. When I try to add a header or footer for the purpose of doing page numbers, I get the message: An error was encountered while retrieving page content. How do I get around this error message to be able to add page numbers to my document?
Something in your PDF file is corrupt, and unfortunately, unless you remove the page that causes this problem (if you can actually remove it), you will not be able to add headers or footers.
If you have Adobe Acrobat Pro, you can try to run the preflight profile "Report PDF syntax issues". This might point you to the correct page. If that's not a possibility, you can try to cut your document in half by exporting pages 1-600 into one file and pages 601 to the end of the document into a second file. Now try to add headers/footers to each one of these. If you are lucky, one of them will succeed, the other one will fail with the same error. Now take the one that failed and cut it in half again and run the test again on both halfs (which are now a quarter each of your original page count). Keep on going until you identify the offending page. Now you can try to remove that page from your original (all the documents you've created along the way are only necessary to identify the offending page) PDF file and replace it with a non-corrupt version. Now you should be able to add headers and footers to your complete document.