I don't think you can replace the eMMC directly (happy to be corrected if I'm wrong).
You can however, insert a SD card (directly above the SIM card) with as big a storage as you like.
I'm not sure how the different distros handle the SD card (automount, mount when accessing in filemanager),
Mobian f.e. mounts it on access.
Edit: There are
eMMC available in the store, but I don't know if you can actually use those with the PP.
And if you actually can there might be some soldering required.
Maybe have a look at the
schematics.
Judging from the
eMMC datasheet a few options might be available,
but again it might not be that easy to actually "install" the eMMC. I'm also reasonably confident that you would void your warranty
In case you are interested in disassembling your device, the eMMC is on
the back of the mainboard. It is the U700 thingy