Adobe is the gold standard for printing for the entire planet. Period. There is no connection between PDF docs and print quality. Adobe prints what it sees. Instead of trying to find blame with a product that prints billions of pages a day globally why not simply try to eliminate the printer itself as the problem? This assumes you have made every adjustment to your printer possible.
ONE POSSIBLE EXCEPTION....before I suggest an approach to this problem I must make you aware of a similar issue I encountered. The docs that were sent to me were scanned. Most docs come from the signing company's computers, they are not printed and then scanned. Scanned files lose some resolution unless your are scanning at 1200x1200 which would take forever. A scanned file may look OK on your screen but print lightly. Are these docs all from the same source?
Of the forms that are printing lightly try downloading that form from it's source (HUD/VA) whatever. Do NOT scan it. Simply try printing it. NO improvement? Printer problem. Improvement? Doc file problem.
Still not convinced? Switch printers. You must have a friend, or at least a print shop, close by. Simply take some pages from the file you received and put them on a thumb drive and bring it to a different printer. NOTE...before everyone starts screaming "you can't do that with non public data", simply use Adobe to pull out only documents without non public data, there are plenty of them in any package, just look for them. I can think of a dozen off the top of my head. If the printer (not yours) prints OK, see above, your printer sucks.