When trying to read a PDF on a bigger screen phone, I like to quickly zoom to a level that maximises utilization of my screen real estate, so that I can see biggest font without having to scroll horizentally. That is to a level that I don't see any of the right or left margins of the page. The app should lock the zoom and horizontal movement when working in this mode. The user can only scroll up/down or jump to a page. I have seen this working perfectly on default PDF reader on iPhone 6+ I have tested multiple PDF readers. The ones I have tested:
Google Drive PDF Reader
Adobe PDF Reader
Drive PDF Reader
Think Free Viewer
PDF Reader by Ivan Ivanenko (is able to lock horizontally, but not easy to use without auto zooming side)
QPDF
I also installed some other popular PDF viewers on the market. None of them had this feature.