A lot of times, it isn't the quality of the PDF, but the quality of the printer. I got the PDF of the manual for my Vermeer wood chipper, and on my printer, the copy is readable, but not that clear. The copy that the dealer printed off for me on their big office color copier is crystal clear, and the pictures are photographic quality. My office laser printer prints in only black, but the quality of the print is excellent, and it is super fast. I also use expensive high-quality paper in that machine.
Ask your dealer if they will print the wiring diagrams for you? They most likely have a better quality printer than the average home-style printer. Most times, printers are set to print to page size, and automatically downsize oversize drawings that should be printed on 11"x17" paper and folded before being inserted into the binder.