It happens even to me (iMac late 2006, OS X 10.6.8, Safari 5.1.7(6534.57.2)), but instead of ??? I get black screens. Here some links to several PDF files:
Hi everybody, maybe I've found a solution for this problem (tip from a forum):
moved the file "AdobePDFViewerNPAPI.plugin" from the folder "/Library/Internet Plug-Ins" to the folder "/Library/Internet Plug-Ins/Disabled Plug-Ins";
left the file "/Library/AdobePDFViewer.plugin" in his place;
launch Safari (my version is 5.1.7) and checked the page of installed plugins (menu Help|Installed plugins). the sectione AdobePDFViewerNPAPI no longer appears in the list.
Now it opens correctly the pdf files within the panels.
Hope this can help you. bye.