Where can a PDF of the TPP legend and supplemental materials be found? When you buy a book of government approach plates, or a Terminal Procedures Publication (TPP) it comes with a legend and other supplemental documents along with it. Now that a lot of pilots are using digital flight bags like ForeFlight it's easy to overlook this stuff.

What I need is a PDF of these materials, but after a lot of searching I'm just not finding it. Does anyone know if the FAA publishes this digitally? This page mentions that it's available, but I don't see the download for it.
Apparently the file you are looking for is FRNTMATTER.PDF, located in the "D" download file.   DDTPPD_201603.zip

(but it still looks like you have to download the 900MB ZIP file just to get that one important file)

The file that @JonathanWalters referenced has very similar information in a much smaller download and a different format.  But there are differences between the two. 

I recommend downloading both the Chart Users guide at the FRNTMATTER.PDF file, and judging for yourself which one (or both) best has the info you are after.