Android - Is there a way to adjust the size of the text when reading PDF files?

Use ezPDF Reader and use its text column locking feature in landscape orientation (to kick out blank margins and use large device height as width of PDF). I find it useful on my 4.0-inch screen even for the codes because it doesn't change formatting to reflow text. But, if your screen size is much smaller than my one, it'd be useless to you.

Plus, it features voice reading. You can use that too.

qPDF Viewer - a free ad-free Android PDF Reader - also has a great text reflow mode. Users can zoom in and out to increase or decrease the font size. Look for "Reflow Text" under the action toolbar "View" menu.

The best option is Foxit Mobile PDF.