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.
PASSWORD="myPasswordHere" pdftk \ input.pdf \ input_pw $PASSWORD \ output output.pdf
sudo apt-get install pdftk.
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
-resize 629. Also, all your images need to have the same DPI, otherwise it doesn't work.