I'm a programmer, you can image how many books I have to read to keep myself updated
Unfortunately, those books are highly formatted with graphics, tables and code snippets.
I understand the "remaps/reflows " but pdf is hard to do that. if you enlarge it, the whole page size will increase as well. So you have to read text from left, then drag it to read right side.
So I guess the answer is no, 7 inch screen is not big enough for A4 pdf file?