I was having the same problem. I am running 10.6.8. Using Safari 5.1.5 , when I want to see a PDF on a website page, first I make sure that when I click on the link, I can see that the progress bar is working and is showing a download for the pdf. For awhile the Safari page that contains the pdf was a dark grey ( almost black ). Control clicking didn't help with any of the options and there was no "Save As" with it. There is a "Save As" under File in the menu bar and when trying that, I get a dialog box saying I can't save this file ( even when I tried renaming it).
So, I followed some other advice to go to the folder with the Adobe plugin in root/Library/Internet Plug-Ins and put into the trash the 2 Adobe pdf plug-ins. You will need your admin password to do this. Then I downloaded a fresh version from Adobe and installed it from here: http://get.adobe.com/reader/
After installing, I refreshed the same page for the pdf with the black screen and it downloaded again. This time the page was a medium grey and when the download was finished the actual pdf opening up. I could then save it using the "Save as..." from the menu bar.
For awhile I copied the url for the pdf and pasted it into Firefox and the same thing was happening. I went to Preferences under Firefox in the menu bar then to the Applications tab. Under "Content Type" near the top was Adobe PDF document -- to the right under Action I used the default at first "Use Adobe Acrobat NPAPI plug-in..." & the DL just had a white page with nothing there. I changed the action to "Save File" and then the DL worked-- I could see that the file was being downloaded when the Downloads window opened. The file was then in my Downloads folder in the Dock. Now after downloaded the new Adobe PDF reader, I wasn't sure if the PDF file was downloaded because the Firefox page was white ( blank ) but I could go to the File menu as "Save Page as", I picked the desktop for saving and the PDF downloaded and I could open it without any problems with Adobe Reader. It worked with any of the "Action" choices in preferences from this point on.
My only conclusion is that the original Adobe Reader plug-in files - there are 2 - one or both became corrupted and downloading the new version fixed getting any PDF in Safari or Firefox.