(Sorry, too long for a comment)
For me, PDF is definitely top of the list. Next would come bitmap formats (GIF/PNG/JPEG) because there's lots of existing schematics in those formats. Some kind of vector format is probably good too if there's a bitmap preview.
I don't like them, but I might add Word/Excel/Powerpoint too because OEMs have a habit of releasing stuff like this (eg the sales presentation that just happens to show the extra components fitted to the pre-production model, or the spreadsheet used to calculate the PLL settings). Word and Powerpoint can be usefully converted to PDF but Excel can't. Also it's useful to keep things in the original format because transcoding can mangle some information (for example, someone who wants to automatically translate a diagram needs to preserve the text ordering).
Perhaps also Zip for miscellaneous files like ROM images and firmware updaters?
I'm ambivalent about EDA formats, but if the OEM releases the chip or board layouts I really want them in a format where I can see the layers - PDF is a poor comparison. And they'll arrive in the format of whatever tool the OEM happened to use. Maybe Zip as a wrapper for miscellaneous other formats, with a suggestion that a PDF/etc copy should be provided too?
