I am working on a PDF library which uses the UserUnit property in some computations. I would like to test the library on real data but I cannot find any real-world PDF documents with non-standard UserUnit. What are the use cases for this property?
You only need UserUnit if you want to specify a MediaSize in excess of (IIRC) 14,400 points (200 inches) in either dimension.
Note that this is an Acrobat limitation, it can't handle MediaSize values larger than that, other applications can. If you want a MediaSize exceeding that, and you expect your file to view in Acrobat, you need to set UserUnit.
I do have real world files which set UserUnit (very, very few) but cannot share them as they are customer files. I'm told that these are used for architectural plans, apparently some regulatory bodies require all the plans to be on a single 'sheet', the only way to do that with (for example) a multi-story building is to have a very large 'sheet'.