Carrying a smartphone has made my tablet obsolete. Digital reading has two main modes; plain text or PDF. Plain text is very much readable on a phone, especially in ‘reader mode’ where the browser extracts the text from the webpage and presents it black on white in a readable font and size.
PDFs are of course a different business. Their text cannot be extracted, instead, you are forced to read it as it is on the page. When the iPhone 6 came out, with a screen of 4.7″, there were some voices that proposed that such a phone could not replace a tablet. My own experience, however, has been different. In landscape mode, and using pinch-to-zoom to make the text of a PDF as big as possible, I find it clearly readable, thanks to the high PPI of the phone. Three other factors weighed in as well. Firstly, my new smartphone has a much bigger storage capacity as my old tablet. Secondly, my new smartphone is (obviously) much more compact and transportable than my old tablet. Thirdly, if the PDF is not on my phone yet I can easily get it as I have an internet connection wherever I go (bar the South Pole and the like). In short: while the PDFs may be tiny on a 4.7″ display (versus 9.7″ on my tablet), I consistently reach for my phone to look at PDFs rather than my tablet.