This is a new security check, added to Adobe Reader.
You can turn it off in the bValidateBytesBeforeHeader registry keys as shown in the link above.
But, be careful - it is possible that the reason your PDF documents are failing the additional security check is because they've been tampered with or infected. This is more likely than you might expect, so be sure to virus scan those PDFs before you go opening them.
Edit: The registry keys given are:
HKCU\Software\Adobe\(product name)\(version)\AVGeneral\bValidateBytesBeforeHeader
- which is a DWORD value, and should be set to 0 to skip the check.
The product name and product version will look something like:
HKCU\Software\Adobe\Acrobat Reader\11.0\AVGeneral
The above key is in the HKEY_CURRENT_USER hive, and will only affect the current user. Another key is located in the same path under HKEY_LOCAL_MACHINE (HKLM).
Also, note that if you have installed a 32-bit version of Adobe Reader on 64-bit Windows, you will have to add Wow6432Node in between Software and Adobe, eg: HKCU\Software\Wow6432node\Adobe\Acrobat Reader...
.