Originally Posted by sadievan
I've been using ES file explorer to add apps. I don't think it copies the apk file. I think it just installs from the file on my computer. At least I can't find an apk file on my Fire.
The Fire will cache some Apps to the Download
folder on the Fire. Use ES File Explorer to navigate to this folder and you should see all App installation packages downloaded. These are not required to be kept after the App is installed. These will eat up valuable storage space on the Fire. The best place to keep the Apps is on your PC and if you ever need to reinstall an App you simply transfer it back, install it, and delete the install file from the Fire.
LONG press (press and hold) any file and you will receive a pop-up menu from which you can delete files individually; or you can use multi-select (+) to select multiple files and LONG press the last one to delete multiple files. If the ES File Explorer toolbar is not visible, touch the small icon in the upper right corner.
Do yourself a favor and get a copy of WiFi File Explorer Pro. This makes it much easier to transfer and manage files using a web browser. The latest version can be purchased from Amazon for 99 cents. An older version can be downloaded here so you can try before you buy: