Two-column papers reflow into one clean column, and you work in that column: highlight, underline and write with a stylus on it, pinch to zoom, and use the whole screen in landscape.
Tap the round button at the bottom right (a book icon), or the Reading Mode tool. The paper appears as one column, with figures and tables as cards in the text. The same button, or the back arrow at the top left, takes you back to the PDF.
Long-press a word, drag the handles, and choose Highlight or Underline in the bar above. They are the same annotations as on the PDF page, and they sync with Mendeley or Zotero.
The side-by-side button at the top, or a long press on the round button, opens the PDF page beside the reflowed text. Your marks show on both.
One column means no zooming or sideways scrolling on a small screen. Change the text size with the buttons or a pinch.
On E Ink devices, Rectoly switches to one page at a time, a footer that always shows, and no bouncing scroll.
Settings › Reading Mode appearance: follow the app, light or dark. Only the reading screen changes.
Rectoly runs on Android phones, tablets and E Ink devices such as Boox. The app comes from this site, and one purchase covers it together with iPad, Windows and Mac.