I ran three PDFs through Ghostscript's /screen, /ebook and /printer presets. Here's what happened to each.

If you've wired Ghostscript into a "compress PDF" feature, you've probably reached for -dPDFSETTINGS. I maintain a small open-source file converter (Flask, Ghostscript and LibreOffice; disclosure: it's my project, KuickKonvert). I wanted to know what the three common presets actually do to ordinary files, so I tested them and published the inputs, outputs and script.

The command

Ghostscript 10.02.1, the same options the converter uses:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen \
  -dAutoRotatePages=/None -dNOPAUSE -dQUIET -dBATCH \
  -sOutputFile=out.pdf in.pdf

/ebook and /printer were swapped in for /screen.

The three inputs

Three things I didn't expect

1. /screen was not the smallest for the photo. /screen downsampled the photo to 720 × 480 px (72 dpi) but stored it with lossless Flate compression. /ebook kept 1500 × 1000 px (150 dpi) but used JPEG, and came out at 58 KB against 495 KB. The photo here is synthetic (generated noise with grain), so a real camera photo may behave differently, but it was enough to stop me assuming /screen always wins.
2. The scan barely moved on /ebook and /printer. Only /screen made a real difference (15% of the original). At 200 dpi, the scan was already close to what the other two presets aim for.
3. A text-only PDF can grow. /printer doubled the five-page text file. My guess is that it embedded the font, but I haven't confirmed that from the output file, so treat it as a guess.

What I do in the app because of this

After Ghostscript runs, the converter checks two things: that the output has the same number of pages as the input (Ghostscript can exit with code 0 and still write a blank one-page PDF for some damaged files), and that it is actually smaller. If it isn't smaller, the user gets their original file back.

The size check, simplified:

import os, shutil
def keep_smaller(src, out):
    if os.path.getsize(out) >= os.path.getsize(src):
        shutil.copyfile(src, out) # compression didn't help: return the original

Takeaways

• Look at what is inside the PDF before compressing: images are where the weight is.
• Don't assume the "lowest" preset gives the smallest file; test /ebook too.
• Always compare the output with the input, and fall back to the original.

Reproduce it

• Files, results and the script that regenerates everything from fixed seeds (CC BY 4.0): https://doi.org/10.5281/zenodo.24199945
• Converter source (AGPL-3.0): https://github.com/khshahid1970/kuickkonvert
• Limitations: each test uses a few generated files. Other Ghostscript versions and real-world documents will give different numbers.