Points for reference:

Display of an A4 page at full size would require approximately a 14" screen. An A4 page has an aspect ratio of approximately 4:3 (actually 4.23:3) so full size wouldn't quite fit on a 14" monitor with 4:3 ratio, but any clipping would likely be absorbed in normal margins.

A printed paper A4 page generally will approximate at least a 150 dpi resolution (granularity) but the typical monitor screen isn't likely to be better than 96 dpi. Older monitors were often 72 dpi, but even new "small displays" may deviate from what's otherwise customary. Resolutions for the devices that have been mentioned probably should be easy to determine - but should and would are sometimes two different things. I haven't noticed any claims being made.

PDFs made directly from text will probably show at near the 96 dpi of the display, so they're likely to look pretty good, but PDF converters that embed graphics into PDF directly from a scanner and then extract text via OCR (built into some PDF converters, but results are similar with separate OCR) generally apply a couple of cycles of file compression, once to embed the graphic and again if generic fonts are applied to the OCR interpretation.

The usual intended resolution in the finished PDF, including the stuff that originated as images, with the most common settings, usually will be near the 96 dpi of the display since most converters are designed with the expectation of computer display (and web posting), but "registration" so that the pixels of the screen line up with the pixels of what's being displayed may give a "pixelated" appearance closer to an effective granulation of as low as 45 dpi - which is likely to look a somewhat fuzzy but may not be noticed except by direct comparison to something done via other methods, especially if the display is a little smaller than the original A4 page.

(The earliest "laser printers" had a stated resolution of 70 dpi, quickly moved up to 100, but we thought 100 dpi print was pretty good for several years.)

If you have a process that gives you images that look good enough on a device you've picked, this is all pretty much irrelevant; but if you run into material that needs to be "done differently" some awareness of where the fuzzies come from could be helpful.

The details of "where the fuzz comes from" will depend on the processes used, but the above referents may help to decide whether improvement is likely by tweaking things, if you need it.

John