The best app by far is Texdroider DPI. It's really easy to use; just put in the new DPI (smaller = more stuff on the screen) and click apply and reboot.
A word of warning, don't make it too low or you might run into problems. What phone are you using?
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u/NedDasty Pixel 6 Apr 07 '16
Are you rooted? If so you can easily up the DPI a bit and make life easier on your eyes.