PDF Cheatsheet

As with all of my cheatsheets, this is just a dumping ground for as things come up.

Save Unlocked PDF

If you get a PDF that requires a password to unlock, you can use this command to save a version that won't need unlocking in future.

qpdf \
  --password=$PASSWORD \
  --decrypt $INPUT.pdf \
  $OUTPUT.pdf

This requires having installed qpdf with sudo apt-get install qpdf.

Create Encrypted PDF

PASSWORD="somePasswordHere"

qpdf \
  --encrypt \
  $PASSWORD $PASSWORD \
  256 -- input.pdf \
 output.pdf

That is not a mistake, one has to provide the password twice.

Create A PDF From A Series Of Images

The following command will create a PDF document where all the images are the same width.

convert -quality 100% -resize 629 \
  "path/to/image.png" -resize 629 \
  "path/to/image2.png" -resize 629 \
  output.pdf

If you want the images to not all be the same width, then remove the -resize 629. Also, all your images need to have the same DPI, otherwise it doesn't work.

Fixing Error Message

If you get the error message:

convert-im6.q16: attempt to perform an operation not allowed by the security policy `PDF' @ error/constitute.c/IsCoderAuthorized/408.

Then do the following. Edit your ImageMagick policy configuration:

editor /etc/ImageMagick-6/policy.xml

Add the following just before the final </policy> tag:

  <policy domain="coder" rights="read | write" pattern="PDF" />

E.g.

  <policy domain="coder" rights="none" pattern="PDF" />
  <policy domain="coder" rights="none" pattern="XPS" />
  <policy domain="coder" rights="read | write" pattern="PDF" />
</policymap>

It should now work!

Split PDF Into Set Of PDFs

If you wish to split a PDF into a set of single-page PDFs, you can do so like this:

pdftk myoldfile.pdf burst 

Pdftk can be installed by running sudo apt install pdftk-java.

Reverse PDF Pages

If your PDF is in the reverse order of what it should be, you can rearrange the pages with the following command:

$INPUT_FILENAME=input.pdf
$OUTPUT_FILENAME=reversed.pdf

pdftk $INPUT_FILENAME \
  cat end-1 \
  output $OUTPUT_FILENAME

Convert PDF To Set Of Images

Tools like Figma make it really easy to export as a single PDF. However, I need to convert that to just a set of PNG images. Luckily this is really easy, and essentially the same as before in reverse (using ImageMagick).

convert \
  input-file.pdf \
  output.png

That will create a set of incremented filenames like so:

• output-0.png
• output-1.png
• output-2.png

Crop PDF

If you have a PDF, and you find that the margins are too large, which is causing it to be hard to read easily on a tablet, you can crop the margins with pdfcrop as shown below:

LEFT=-20
RIGHT=-25
TOP=-60
BOTTOM=-175

pdfcrop \
  --margins "$LEFT $TOP $RIGHT $BOTTOM" \
  /path/to/pdf.pdf

This can also be useful for when printing if you want to "zoom in" on the content as the printer can scale to fit the width.

References

• Stack Overflow - How can I easily crop a PDF page?
• How-To Geek - How to Remove a Password from a PDF File in Linux
• Stack Overflow - ImageMagick security policy 'PDF' blocking conversion
• Linux Commando - Splitting up is easy for a PDF file