So you want to modify the text of a PDF by hand...
If you, like me, resent every dollar spent on commercial PDF tools,
you might want to know how to change the text content of a PDF without
having to pay for Adobe Acrobat or another PDF tool. I didn't see an
obvious open-source tool that lets you dig into PDF internals, but I
did discover a few useful facts about how PDFs are structured that
I think may prove useful to others (or myself) in the future. They
are recorded here. They are surely not universally applicable --
the PDF standard is truly Byzantine -- but they worked for my case.