Since an update about one or two years ago the possible zoom factor for viewing PDFs on Android devices has been limited to about 200%.
It was at least 400 or 500% before. I was hoping, that the devs will fix that, but unfortunately nothing happened ever since. Watching architectural plans or similar detailed PDFs is almost impossible now on mobile devices
If anyone is unhappy about this issue as well, please give this post a like, so the evernote dev team might give this issue priority.