PDFs, unless A5 format, need either paper, laptop or a Tablet. A small subset work on 300 dpi eink 6" models. The 7" models are a bit better.
Sometimes reconversion or cropping or colour depth or brightness/contrast helps using The Gimp (import to set DPI and export in reverse order as mng file) and ImageMagick (mng to pdf with crop and bit depth and brightness/contrast) or use
K2pdfopt