Custom imported maps are stored local to the app and may not be visible through the system directory. When a geospatial PDF or GeoTIFF is imported into the app, it is no longer in that format, as it is essentially broken into a series of tiles to allow for zooming. Because of this, and to prevent corruption to the files, the location is kept internal to the app.
However, prior to importing, you can choose to specify where you want to files to process and be stored, at which point you can locate those processed files. To do this, please open Avenza Maps and go to Settings > General > Specify Map Folder. Once you do this, any custom imported maps (non-map store maps) will be stored and processed in this location.
I hope this helps answer your question, but please let me know if you do have any further questions or problems!