As a general rule, Acrobat ignores and silently fixes a lot of issues with PDFs that more stricter implementations will complain about (it goes beyond the spec to be accommodating). This unfortunately means a lot of programs out there are making malformed PDFs but their users don't know because "it works here on Acrobat!". So that's one reason I have to install it despite alternatives on Windows, but maybe Preview is the same in this regard and fine for general users.
