Although it is possible to convert PDF's to the one of reflowable formats, the results for textbooks (formulae) and technical specifications (diagrams) are poor, IMHO. Considering the wide popularity of A4/letter size PDF as the publishing format for those documents, I believe that the proper solution is a device that can display an A4 (or letter) PDF's, with a zoom function for details.
At the time that Kindle DX was introduced, sirbruce was kind to capture a screenshot for me. You can see the results
here.
Based on that, and if you share my view about conversion from PDF, you will need a bigger eInk screen to display A4/letter sized PDF's. The price of these devices is still relatively high, but we are getting there.