[Frank Cox]

On my (linux) computers I keep pdf files in a compressed format, usually using either bzip2 or xz.

So my pdf files have filenames something like this:

songbook.pdf.bz2
songbook-2.pdf.xz

This can save a significant amount of space since a pdf file compressed in this way will usually shrink by anywhere from 10% to 50%, depending on the makeup of the original file, i.e. how compressible it is.

The pdf file readers that I have on my computers (atril and evince) are smart enough to automatically de-compress a compressed pdf file before rendering it to show me.  Under the hood, what happens is that the pdf viewer recognizes a bz2 or xz (or gz or what-have-you) compressed file and shells out to the appropriate decompression program to put the decompressed file into a temporary directory, the temporary file is file is rendered for viewing and the temporary file is deleted when the viewer is closed.

This does add a bit of time to the initial rendering of the compressed file but it saves a significant amount of space on your hard drive/card/ram/whatever.