I often use my smartphone for prolonged periods of time (reading or drawing) and my vision gets noticeably blurry as a result (I once needed more than an hour to get it back to normal). I've read this is only temporary and does not cause any permanent damage but it's annoying and I would take any chances with that anyway.
Things I've done about it:
I use a dimmer and a blue light filter.
I optimise my pdfs for less eye strain (type, font size, background colour).
I also know about the 20-20-20 rule but I find this really impracticable - I try to take breaks once in a while but breaking the flow of whatever I'm doing every 20 min. is just not something I'd do.
Is there anything else that I haven't considered?