Rather than making a COMPACT version, your command created a file over twice the size of one created using the aforementioned
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.
If you lower the resolution those auto-scanning services that does OCR on the documents will get problems converting the tex back to normal text. Which might or might not be the point of doing this in the first place... :-)
Re:Show HN: Make Your PDF Look Scanned
by mwcampbell on Friday May 15, 2020 @08:32AM
(#23191383)
Just remember that if you make it harder for OCR to produce accurate text from an image, you also make it harder for blind people to read it.
The output file size relative to other approaches will depend on the contents of the input file (e.g., does it embed any bitmaps, or is it only text in non-rasterized fonts?), the density you use (e.g., 150 dpi is 4x greater than 75 dpi), the bit depth you use (3 bits is 4x greater than 1 bit), and whether you add noise or not. Also, the one-line command above your comment is short/easy to remember and reasonably fast -- which is useful if you're impatient like me.
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.
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.