That cr*ppy pdf
I don't think that it's a legal requirement; that would be covered by the existence of the paper document, if necessary.
No, it's the appalling and inconsistent state of the MOD's (not just the RAF's) IT kit. Not many people have a way of making pdf files and they are probably unaware that it would come out much smaller and neater if it were straight from the original file – electronically.
If only the MOD had continued with Macs ...