[Maarten]

PDF itself behaves just like a container to put in JPEG or PNG files, no conversion or recompression needed as those are the most relevant supported internal formats (PDF also natively supports vector drawings, which would be ideal but converting a scan to a vector drawing isn't feasible, especially for historical/archival purposes). Of course the software you use to create the PDF could be equipped to further compress JPEG (technically fully possible, by the way) or PNG with inevitable loss of quality.

The problem with further compressing JPEG is that it will give quality loss in such a way that it's particularly noticeable in line drawings and text. Fine details in PCB drawings (with grey areas) will get lost at some point as JPEG considers them details in a photo and treats them as such.

The best way to avoid all of this is indeed to convert highest possible quality source material (including JPEG except photographs) to PNG before making a PDF. I prefer to do all conversions and editing on the raw material before putting it in the PDF container, but this can be done with dedicated PDF editing software of course. Still, it is useful to know what happens inside.

I consider archival quality to be 300 or rather 600 dpi, but 150dpi is fine for most materials for general distribution