pdf is ok. Recommend you also make available the source svg file and scans/pics so that:
- if people want to change background color to make aged paper look, it's easy without finding the right color card stock
- if they have a same era game, making the cards for another machine is a lot faster
- add/change replay values
- if someone just supplies a scan/pic, someone else can make the svg file
it'd be great to have a place for font downloads too.
inkscape svg files contain some things that are svg 2.0 and other stuff that is unique to inkscape. The files may not render well in browsers or other programs, but since inkscape is open source, free, and supported on many operating systems, it's a good tool for collaboration.
final suggestion is have a "wanted" place on the site. The pacific pinball museum has 1000+ games in their warehouse and I can occasionally look and see if original cards are on them. I prefer scanning the cards so the best reproductions are possible.