I'm indifferent to which format is used.
PDF is nice because it's one file. Good compression for B&W manuals and everything. But it has it's own shortcomings to pull it down.
In contrast JPG is a lot of files which is both an advantage in case you only want a certain part of the manual, but as was said. JPGs don't do that well for B/W scans and I believe they might even take longer to scan than PDF even if the PDF need to scan over a page twice.