Update in 2020:
PRC is a old Kindle format adopted in the Kindle books from early Kindle Android app. From Kindle for Android V7.2 and later versions, all the downloaded Kindle files are KFX format, not the old PRC format.
And currently there isn’t any effective method to directly remove DRM from the Kindle KFX books via Kindle Android app (including Kindle Fire tablet).
The solution is to download Kindle books via Kindle for PC/Mac, or via Kindle Cloud Reader. For details, please go directly to the page — 4 Methods to Remove DRM from Kindle KFX eBooks in 2020 [Video]