This can be if the file is not correct. Simply click OK, if the file is not password protected. This often works because the next tool can maybe handle this PDF.
You can suppress this message if you set the following property in the registry:
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\PDFPrint
HKEY_CURRENT_USER\Software\PDFPrint
ignorePdfUnlockFailed [DWORD] = 1
So you have to set the ignorePdfUnlockFailed option to 1 in the registry under one of the above paths. Then the message is skipped.