I have neve had an issue with navigating to an online PDF file before, and yet now with the OS update neither Safari or Firefox can open PDF's in the browser.
Open a Finder window then from the menu bar click Go > Go to Folder
Type or copy paste the following:
If you see the: AdobePDFViewer plugin in the Internet Plug-Ins folder, move it to the trash.
Restart your Mac and try opening a PDF file.
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