Comment 6 for bug 1527884

Actually Ffx (per #5) can download PDFs here, it just can't open them in anything other than its own preview. But it won't download and save files.
If I set Prefs > Apps > PDF to Always Ask - it doesn't, and the download history just says 'Failed'. Same if I set it to open in Okular or Adobe Reader. But 'Preview in Firefox' does work OK. Unfortunately Firefox's poor printing doesn't work well enough for the shipping labels so we needed to open them in Okular first and print from there.
This machine also fails to load PDFs from local files and open them in anything other than Firefox Preview - again it shows them under downloads as 'Failed - local file'.

One difference between the 1404LTS machines where PDFs can be opened in other apps (see #5) and the machine where they can't, are the Prefs > Applications options for PDFs:-
PCs where app options work OK list 3 options for PDFs, including 'PDF document (application/x-download) - the relevant one here.
PC where app options don't work (only Preview in Firefox) only offers: 'Portable Document Format (PDF)'.
I tried deleting the mimeTypes.rdf file so it got recreated, and copying it from a machine that works, but neither helped.