This is from the article noted below.
Acrobat products have historically opened a PDF as long as the %PDF-header started anywhere within the first 1024 bytes of the file. No checks were performed on the extraneous bytes before the %PDF-header. However, the 10.1.5 and 11.0.01 updates improve security by enforcing stricter parsing of the PDF-header. The product now refuses to open the small fraction of PDFs that do not correctly start with the '%PDF-' header and an error appears:
Error: the document is damaged and cannot be repaired. Adobe Reader could not open because it is either not a supported file type or because the file has been damaged (for example, it was sent as an email attachment and wasn't correctly decoded).
There are a few workarounds mentioned in the article.