git "too hard to use"?

It's impossible to merge them *automatically*. This is a flaw in the file format and toolset, and has nothing to do with version control per se.

If you have more than one documentation writer on the payroll, you're already spending time merging changes manually. Even if you make each chapter (or whatever) of documentation a separate file, there will still be some cross-references and dependencies.

If you would have used .tex and .svg, you wouldn't have this problem, or at least not to this extent. If you like Microsoft, their Sharepoint stuff is designed to solve exactly this problem-- to make office documents mergeable.

It's hard to see how you can get away from using subversion and mandatory locking with the workflow you have now. But this is not because employees are too stupid to understand branching. It's because the tools you are using are old and rusty, and don't support the diff operation. This also will make it hard for you to audit what people have done. If I make a few spelling corrections to the PDF and then commit it as "added whole new section on zebras" how is my manager going to know the difference? You're just using the honor system, basically.

By the way, if you need mandatory locking and good handling of binary files, you might also check out Perforce. They have good GUI and Windows support (although subversion has been improving a lot lately.)