I'm using Vista 64,and updated to Reader X. It will open standard .pdf files, but secure ones (I downloaded manuals for my Canon digital camera software) not only will not open, but cause Reader X to stop responding.
My temporary solution: delete X and reinstall 9.4.whatever.
What has happened to the infallibility of this software?
It could be that there is a problem with the pdf files. Each version of Acrobat and Reader are more intolerant to malformed pdfs as that is a vector for spyware and virii.