While I don’t have an App recommendation, the feature you are looking for is mostly called “text reflow”.
It works a bit against the intention behind the PDF format, i.e. to always present document pages exactly in their original layout. But PDF viewers with text reflow do exist.
Be aware that some PDF documents might not include any plain text but only graphics (perhaps showing text).
Text reflow might not work with such files if the PDF viewer has no built-in OCR feature to try to recognize text in graphics.
Trying it out by opening a couple of PDFs, and it looks like it does not support reflowing text. On the documents I tried, it opens the file at a zoom level that shows the whole page. Zooming in to make the text bigger does not reflow the text
Edit2: Back on the computer, I tried a few things. Didn’t get very far with LibreOffice Viewer. Collabora Office displays ODT files just fine, but without text reflow … unless you enable edit mode and alter the page width .