For a long time I’ve wanted to be able to add math margin notes to a PDF. With OS X’s PDF-based display, this seemed like it should be possible, but Acrobat doesn’t do it. It cannot accept PDFs that are drag/dropped or pasted. I was even considering writing some tool to do it, but I found the way: a combination of LaTeX Equation Editor (free, great with Keynote) and PDFpen. With these two programs, you can type LaTeX into LaTeX Equation Editor, then drag-and-drop the resulting mini PDF onto your PDF in PDFpen, resize it there, and save. [sample below]
One caveat is that PDFpen has to be in pointer mode or have most recently used the insert-image-or-PDF button; if it’s in scribble or other annotation mode it does not accept the drop.
Nowadays, most of my knowledge of complex mathematical equations comes from watching the (improbable but entertaining) TV show NUMB3RS, but the tip itself made sense to me.