Why do I get gobbledegook printed when the print preview is OK? (This is only with documents downloaded from a particular souce)

One thing to check and try. Do you close the PDF document before it's finished printing? If so, try leaving it on screen until the last sheet is printed.

What version of Reader?

what system?

What sort of gobbledegook? For instance, enfless pages of code? The pages with pictures but the text sort of randomised? what?