Hello, I'm running 10.9.4 on a Early 2013 Retina MacBook Pro. I can not view PDFs using Quicklook or Preview.app (though all other supported file types work fine). The file's icon is just a blank sheet. When I hit spacebar on it, a small window appears with that blank icon, the file name, and file size. If I double-click the file, Preview.app opens but displays NOTHING. Furthermore, if I try to open the file within Preview.app using File>Open, all PDFs are greyed out. I had Adobe Reader installed, which has no problem opening these files. Wondering if there was a possible conflict, I used CleanMyMac 2 to uninstall Adobe Reader, but that did not help. If I go to "Get Info" on a PDF and delete the .pdf from the filename, Preview will open the file, though Quicklook still does not work. I've tried many things to troubleshoot, including: 1) Going to "Get Info" and making Preview.app the default PDF reader. Adobe Reader initially was, and I noticed that when trying to change it to Preview.app, it was not listed as recommended application, so I had to select it instead from "All Applications". Believing Adobe Reader was trying to hijack all my PDF reading, I then went about... 2) ...Deleting Adobe Reader. 3) Repairing permissions 4) Rebuilding the launchservices database 5) When double-clicking a PDF file, I closely watched Console. I noticed a message reported by gkbisd that the cdhash could not be collected for QuicklookUIHelper and Preview.app. Believing gkbisd to be related to Gatekeeper, I changed my Mac's security setting to allow "All Applications." It didn't work. But it made me... 6) ...Wonder if the OS was glitching out, so I installed the 10.9.4 Combo Update. It resolved the Console messages reported by gkbisd, but not the problem itself. I couldn't see any other suspicious messages in Console. 7) I booted into Safe Mode. Here's where I'm perplexed. I immediately went to a PDF file in Finder in Safe Mode and sure enough, Quicklook and Preview.app both worked. After about 20-30 seconds, though, the problem reared its head again. And what's more, shortly thereafter, I found myself logged out (I don't know if that's related). I saw nothing suspicious in Console until I was logged out. I have no idea what's going on. I appreciate your help.