Is this with opening a direct link to a PDF file in a Firefox tab?

If this isn't working then the server might not send the PDF file as application/pdf.

• https://support.mozilla.org/kb/change-firefox-behavior-when-open-file

See also:

• https://support.mozilla.org/kb/view-pdf-files-firefox-without-downloading-them
• https://support.mozilla.org/kb/disable-built-pdf-viewer-and-use-another-viewer

Can you post a link to a publicly accessible page (i.e. no authentication or signing on required) where you see this issue?

http://ingramswaterandair.com/gwh18tcd3na1ao-outdoor-unit-gwh18tcd3nai-indoor-handler-p-19711.html

Above is a link. Scroll down to the download section and try Terra HP Owners Manual

Clicking on the link to their Pdf file does nothing. I have experienced many sites with this problem. Again if I use Internet Explorer I am always able to get to the pdf

No problems here with the PDF links under both sections. They open for me the built-in PDF Viewer in a tab.

You can set the pdfjs.disabled pref to true on the about:config page to disable the build-in PDF viewer.

You can check the value of the plugin.disable_full_page_plugin_for_types pref on the about:config page and remove the application/pdf part if present or reset the pref to the default via the right-click context menu if you want to display PDF documents in Firefox with another application (i.e. not the built-in PDF Viewer).

Under the convert web pages to PDF tab: Options I selected Use PDF Plugin instead of Use OS Default viewer. That fixed it.

Thanks for getting me going in the right direction.