Why can't I markup text in PDF-A in Acrobat X?

I am using Acrobat Pro X. I have a PDF file (Version 1.4, Acrobat 5.x, created Dec. 2009). When I open the file, I see the message, "The file you have opened complies with the PDF/A standard and has been opened read-only to prevent modification."

I want to markup it up--highlight, comment, underline and so forth. I cannot do this. The Tools are grayed out and, as the message says, the file is read-only. It is an unsecure file (No Security). The file creator swears this should not be the case and the file should be accessible to me. Is there something wrong with Acrobat Pro X or is there a compatibility issue?