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:
<code>convert scan.png -quality 50 scan.jpg</code>
You can add ''-level 10%,80%'' to make black more black and background less gray.
Finally use:
<code>convert scan.jpg scan.pdf</code>
to convert your scan to a pdf. You can give ''convert'' multiple JPGs to create a multi paged PDF.