to scan something on KDE you can use scanlite. If you scan a A4 page in color with 300dpi the file will be a PNG of about a dozen MB. To convert it to a jpg use:

convert scan.png -quality 50 scan.jpg

You can add -level 10%,80% to make black more black and background less gray. Finally use:

convert scan.jpg scan.pdf

to convert your scan to a pdf. You can give convert multiple JPGs to create a multi paged PDF.