A PDF is a picture of a page and a .txt is a wall of lines. On a phone or an e-reader both mean pinching, scrolling and losing your place. This turns either into a proper EPUB that reflows to your screen — and it does it here, in this tab.
It reads the real glyph positions in the page rather than the text dump most converters use, which is what makes the difference between a book and a wall of text. Running heads and page numbers are dropped. Words hyphenated across a line break are rejoined. Chapter headings are detected and become real chapters, so a contents list works and your e-reader can jump around.
It cannot help with a scanned PDF — pages photographed rather than typeset hold no text at all, and it will tell you so rather than hand back an empty book. Those need OCR first.
Heavily designed pages — textbooks, magazines, anything in columns with figures — will lose their layout. That is the trade: reflowable text cannot also be a fixed page.
Most .txt books come from Project Gutenberg, and they need more care than splitting on blank lines. The lines are hard-wrapped at about seventy characters, so they are rejoined into real paragraphs rather than left as a column of fragments. The licence boilerplate at each end is removed. The title and author are read from the file itself, not guessed from the filename.
The fiddly one is the contents list: every chapter heading appears there before the book starts, and a converter that does not notice gives you two dozen empty chapters at the front. This spots it by spacing — real chapters are hundreds of lines apart — so it works whatever language the word "Contents" is in.
Those are already reflowable books, so there is nothing useful to convert them to. Ilusi opens all three directly, and so do most e-readers. Kindle has taken EPUB since 2022; MOBI is the format Amazon itself retired, which is why nothing here produces it.
The EPUB is yours. Send it to a Kindle, open it in Apple Books or Kobo, keep it. It is an ordinary file with nothing of ours in it.
It is also exactly the file Ilusi would have made for itself — same reflow, same chapter detection — so if you do open it in Ilusi, illustrations anchor to the right paragraphs. That is the only connection between the two, and it costs you nothing either way.