I have a document that recipients are expected (or are likely) to print on their own. It is not clear whether the majority of users will be US or non-US, so there isn't a clear preference for letter or A4 paper.
Assuming that I can't know how many recipients will want letter and how many will expect A4, what would be the more appropriate page size to use for a PDF?
To put it another way: which will cause less issues for users:
I've found it best to target letter height and A4 width - i.e. the lowest common denominator solution.
Unless you have a very strong US-biased group of clients, use A4. A4 is an international standard, while Letter is only used in the US and Canada.
It is also not necessarily true that "someone using an A4 filing system would have no problem fitting the smaller letter sized into the system" as Scott claimed, since Letter is wider (215.9 mm) than A4 (210 mm).