I recently had to figure out how to get about 4G of pdf files off my phone and onto my PC. Tried connecting the phone to the PC via USB, but discovered that unless the storage card is formatted as external storage, not all files will be accessible via USB. Some, most notably the ones I wanted to transfer, are not available to access if you have formatted the card as internal storage for your phone.
I ended up finding them on the phone with ES File Explorer (they were hidden in the application folder of the app where they were acquired from and not in my Downloads folder), then uploading them to Google Drive, which then automatically copied them to my PC, so that I could then delete them from my phone with ES File Explorer, move them to a different folder on the PC, and then delete them off my Google Drive.
And I really do have to thank Inspira Technologies, makers of my inexpensive Astrotab A10 tablet
, for introducing me to ES File Explorer. It came preinstalled on the tablet and I liked it so much that I installed it on my phone.