As you install an Adobe Reader and Adobe Acrobat application on a Mac machine, the installer additionally installs a NAPI plugin inside Library -> Internet Plugins Folder, so that Mac users can open PDFs inside Safari using Adobe Reader/Acrobat app seamlessly (instead of using the conventional Preview which only supports a small set of PDFs).

But For Mac Snow Leopard users, the upgrade of Safari to 5.1.9 has proven fatal. Till Safari 5.1.8, users will be able to still open PDFs inside Safari without any issues. But post upgrading Safari to 5.1.9, Adobe Reader do not opens PDFs inside Safari.

As of now many users have reported this issue on Apple Blogs: https://discussions.apple.com/message/21880385?ac_cid=tw123456#21880385

Some users have reported this issue via Bug to Apple as well (rdar://13694491)

To resolve the issue, I hereby give our Adobe users a quick workaround:-

Basically, the earlier versions of Safari used to have an entry in the com.apple.Safari.plist for WebKitOmitPDFSupport which now do not exist anymore. So we just go and add it.

To add the entry:

1.)    Go to Library -> Preferences

2.)    Open “com.apple.Safari.plist” in Property List Editor or any Text Editor like TextWrangler.

3.)    Add a new key with the name “WebKitOmitPDFSupport” of type Boolean and define its value as True (checked). i.e.:

<key>WebKitOmitPDFSupport</key>

<true/>

4.)    Save the plist.

5.)    Restart Safari.

After applying the above workaround, users will be able to open PDFs inside Safari again. Note that, in case if you don’t have Adobe Reader / Adobe Acrobat installed on your Mac, then the above workaround will simply download the PDF onto your system.

Just for reference, Adobe fixed the same issue earlier in 2011 for Safari 5.1.0 : http://blogs.adobe.com/acrobat/compatibility-advisory-regarding-adobe-reader-plug-in-and-acrobat-plug-in-with-safari-5-1/

http://helpx.adobe.com/acrobat/kb/pdf-plug-doesnt-load-safari.html