For A4 PDF documents, I prefer using my good old Galaxy Tab 10.1 tablet. The screen size is big enough to avoid frequent zooming and panning without straining my eyes. I also own a old Nexus 7, which works for quick reads, but for longer A4 documents, I consider its display too small.
So if you can get your hands on a used Galaxy Tab 10.1, revive it with a modern CyanogenMod 10.1 firmware, you have an excellent and inexpensive e-book reader for that kind of content.
