I just defragmented my HD and now I can't open certain pdf files. How do I fix this?

So here's the deal. I was defragmenting a HD partition the other day, and it said I only had 8% free space, and that the program needed at least 15% to run correctly. I moved a couple of folders to a different partition, defragmented that one without getting another free space warning (apparently I had freed enough), then moved those folders back. Now, when I try to open certain pdf files on that partition, nothing happens! These files weren't in the folders I had moved in order to make space, they remained on the partition during defragmentation. I don't know what's wrong with them, but I really need these files! If I right-click the file I don't get the normal right-click menu I get on other files. It's much shorter, it doesn't have all those options. If I choose 'Open with' and then Adobe Acrobat, it says 'There was an error opening this doent. Access denied.' I can't rename, cut or copy the file either. Restarting my PC doesn't change anything. It worked just fine until today!