A family of Microsoft word processing software products for creating web, email, and print documents.
Well, Microsoft really doesn't have anything to do with it :-) Since generating a PDF actually is a print process, there are 2 primary considerations involved;
- Word processing documents typically have margins on all 4 sides. Content within the margins is treated by the printer as being in the printable area. Content outside the margins is not treated as being printable. This is a standard, not something that is a part of The Great Microsoft Conspiracy. However...
- Even if margins are set to zero, most desktop printers (including PDF creators) will not print to the edge of the paper (what is called "full bleed") unless the print job is specifically set to use a Borderless Paper Size setting.
To "get rid of them" you can either use Page Setup in Word to specify something such as US Letter (Borderless) or make the specification in the Print dialog. Options may vary depending on OS X version as well as printer model.