JASONS  Many people who want to save months of time in writing/editing their tech documents are heavily replying on MathMagic solutions all around the world.

They often save tens of thousands of $ or make more money based on the higher productivity.

If you don't have to deal with lots of equations or you don't have to worry about any deadline, there will be many other cheap approaches.  
(Version 5.74)

JASONS  Just great. Works well with my Keynote.

MathMagic generates the best quality equations in most cases(I've tried at least 5 of more Math software). Better than LaTeX equations.

Their free upgrade service is also a big plus.

Play with the Free trial version. It's worth if you are dealing with equations in your PDF or printing material.  
(Version 5.3)