Reading a journal article on a phone usually means pinching, dragging sideways, losing your place and giving up. The text is not too small. There is simply too little of it on screen.
A phone fits the whole page width on screen, margins included. On a typical article the margins are about a fifth of the width, so a fifth of your screen is showing blank paper and the text is shrunk to fit what is left.
Zooming in makes the text bigger but pushes the page wider than the screen, so every line now needs a sideways drag. Trimming the margins instead lets the reader fit the text to the screen, which is the thing you actually wanted.
It is worth being a little conservative. Page numbers, running heads and footnotes live closer to the edge than the body text, and a crop tight enough to remove all the white will often take those with it.
A trim of 10 to 15 percent is usually the sweet spot, and it makes the text noticeably larger at phone width. Turning the phone sideways gives a wider line still, which is good for papers with small type.
If a document is really just text, Extract text gives a plain version that wraps to your screen at any size. It has no layout, which is exactly what makes it comfortable on a phone, and you can paste it into any notes app.
Yes, and the effect is even more noticeable, because each column ends up filling the screen.
The original is never changed, so keep it. The cropped copy is a separate file.
Yes, and scans usually have the widest margins of all. Check a few pages, since scanned pages are rarely aligned identically.
Extract text produces plain text that wraps to the width of the screen. It has no layout, tables or pictures, which suits reading but not every document.