I discovered that I was also unable to view PDFs in Safari. Hadn't really bothered me until a poster in another forum questioned why he couldn't. I suggested that he needed some Plug-In. To make a long story short, the more recent version of Safari already have that Plug-In. BUT, if you ever install Adobe Reader or Acrobat, you'll probably end up with it taking control of viewing the PDFs. And I've seen that dieabled checkbox several of you have mentioned. Nasty programming by Adobe, in my humble opinion!
I was able to remove that check mark by simply deleting the Plug-Ins that Adobe had installed (mine were not in my ~/Library but the main Library/Internet Plug-Ins). Unfortunately, even restarting the Mac or Safari didn't change anything...well, I could now get a white blank screen instead of a black one but that wasn't too interesting, either way! 😝 The problem is that while Adobe set a vartiable to a value, deleting its Pug-In(s) doesn't re-set the variable. That's what has to be done.
In an attempt to re-set that variable, someone posted a Terminal command very early in this thread:
1. defaults write com.apple.Safari WebKitOmitPDFSupport -boolean-neg
as suggested above. Then closed and re-launched Safari.
No apparent affect, but probably no harm done.
The problem is that "neg" is not a valide Terminal value for a Boolean (or "bool"). And if entered exactly as posted, it actually puts the value "-boolean-neg" into the .plist file. Amazingly, that doesn't seem to cause any problems, fortunately. 😀 The appropriate values could be any of the following: true, yes, false, no. The first two are equal to putting a 1 (one) in the .plist. The other two values place a 0 (zero) in there.
But, do you see what the name of this variable is?! WebKitOmitPDFSupport. What this means, therefore, is that IF we want Safari to display a PDF the value must be false/no/0 (zero). In other words, it's a double negative! 👿 You can view what's in that variable in Terminal by using:
defaults read com.apple.Safari WebKitOmitPDFSupport
If it is anything other than zero, you will NOT be able to view a PDF in Safari. So, the Terminal command to restore that capabilty is:
defaults write com.apple.Safari -bool false
You can also make this change by opening the com.apple.Safari.plist in a TEXT editor (not TextEdit or Word or Pages!) like TextWrangler. Then, search for something like "WebKitOmitPDFSupport" and you should find only one occurance looking similar to this:
<key>WebKitOmitPDFSupport</key>
<true/>
Now, change the "true" to "false" and you should be able to view a PDF in Safari, again! At least until you re-install something like Adobe Reader/Acrobat/etc.
Finally, use the Safari->Help->Installed Plug-Ins and look very, very carefully for "pdf" in the list (probably in the 'extensions' column. Anything that has that entry will probably negatively affect Safari's PDF display capability! For example, Speed Download "intercepts" any .pdf file and forces it to be downloaded! I had to deselect that setting in Speed Download.
I hope this helps what seems to be a Battle of the Bits!! 😮