I am using Mac 10.10.5 Yosemite
Background task: “Failed to Export PDF file”
I quit and restarted InDesign.
When I created a new file, I could save the pdf.
There seems to be a problem with my file but I can’t figure out what.
I ran into this very problem recently. There's probably a graphic (or possibly a font) which is corrupted.
Try exporting the first half of the pages to PDF, then the second half. If one works and the other not, then the problem lies in the problematic half. Then break that half in two and export out half of the pages each time to isolate the page. Then delete graphics until you see the problem graphic. Resave the graphic file.