I bought an iPad specifically for reading PDFs. It does get used for other things too, but the primary driver was PDFs. Most tablets seem to be optimised for horizontal video content, which makes then too narrow for A4 PDF, whereas the 4:3 of the iPad is perfect.

I purchased a Supernote for this very reason. Not cheap, but I (unfortunately) read a lot of PDFs.

Some software has PDF reflow, like koreader.