Larry,
What are the margins normally set to for an A4 document?<<
There is no "correct" answer to that question. What is the intent of the PDF's? How much white space does the design require?
The 'absolute' limit depends upon the printable area of the printer it is being sent to, for one thing, but the 'average' printable area for an A4 Laser printer will be about 200mm x 290mm and would be centred on the A4 sheet. For most inkjets it would be less than that with uneven margins.
If printing is not your concern, then it becomes purely a design issue. Some designs require lots of white space, others very little.