Recent versions of the Mac OS X web browser Safari do an excellent job of displaying PDF files inside the browser window, much better than the Adobe Acrobat Reader plugin or standalone PDF viewers. Assuming you agree with that sentiment and want PDFs displayed in Safari, you might discover that upon installing some Adobe products (I presume Acrobat specifically), Safari no longer does its own thing. In my case, not only did an Adobe product install a plugin that subverts the built-in viewer, but the plugin fails to function, complaining it can't find Adobe Acrobat Reader.

To disable the Adobe Acrobat Reader plugin for Safari (working or otherwise), find and delete /Library/Internet Plug-Ins/AdboePDFViewer.plugin, then restart Safari.