Unfortunately, your requirements are contradictory: no 5" device will show PDFs well, unless those PDFs have been specifically formatted for the small screen of the reader. Otherwise there are only two choices: to display the PDF full size and "pan" around it, or to attempt to "reflow" it (which will make all the equations, etc, disappear). Neither is a good solution. If you want to display PDF textbooks, etc, you want a reading device with a LARGE screen; the most affordable of these is perhaps the iRex DR800.
This is, please note, a world-wide forum, so saying "<250" is meaningless. Less than 250 what? 250 Turkish lira will buy you somewhat less than 250 British pounds, for example