convert -density 150 ORIGINAL.pdf -colorspace gray +noise Gaussian -rotate 0.5 -depth 2 SCANNED.pdf
Also consider using `-density 100` or even `-density 75` for long text documents. Using a density of 75 dpi produces documents that are 4x smaller than 150 dpi (75²=150²/4) and doesn't affect the readability of normal-sized (10-12pt) text that much.
Finally, sometimes it works best not to add Gaussian noise.
convert letter.pdf -colorspace gray \( +clone -blur 0x1 \) +swap -compose divide -composite -linear-stretch 5%x0% -rotate 1.5 as-scanned.pdf
However, that may be a useful feature, since many users end up inadvertently creating very large PDFs when scanning.
All that said, note that you can always add a `-compress <compression method>` to the convert command, although this could make it noticeably slower, say, for a directory full of long documents. There's even a Fax TIFF compression method that makes the document look like it came out of one of those fax machines you see in movies from the 1980's. To get a list of supported compression methods, type `convert -list compress` at the command line.
convert letter.pdf -colorspace gray \( +clone -blur 0x1 \) +swap -compose divide -composite -linear-stretch 5%x0% -rotate 1.5 as-scanned.pdf
[0] https://news.ycombinator.com/item?id=23157979
[1] https://tex.stackexchange.com/questions/94523/simulate-a-sca...
To be honest, I'm waiting for a boom of such services, that can be run in a separate tab without any network jumps.
IF EXIST scanThis.pdf (magick convert -density 100 scanThis.pdf -colorspace gray +noise Gaussian -rotate 0.5 -depth 2 SCANNED.pdf) ELSE (ECHO File scanThis.pdf not found & PAUSE)