Moving data to micro USB memory?

[Stephen Murt]

Have an M9 and love it. I sometimes need to look at large files of maps and lay outs. Rather than keeping data on my phone I want to keep it on a memory stick (Sandisk 64g ultra dual flash drive for OTG .)
Only trouble is, I don't know how to look at the files I have on the stick! I know I go to File Manager, but then what? Please... Keep it simple, I'm not too savvy with this stuff. Thanks in advance!