I downloaded a file (PDF) from email on my Lumia 730 with Windows Phone. As soon as I downloaded it, I was able to see the document in Adobe PDF Reader. But where is the file stored? How do I delete it from phone?
If you're using Windows Phone 8.1, you can download "Files" app from the Store and search for your file in Downloads directory on Phone or SD card.
If you're on Windows 10 Mobile, you should have the "File Explorer" or something like that already installed on your phone, which is meant to work the same way.
And the last way to find your PDF file is plugging your phone to your PC using USB and searching for it with Explorer app.
EDIT: I think, that you should have the possibility to remove PDF file in Edge browser by long pressing downloaded file and selecting "Remove", but I don't remember if there was such thing.
With the Edge browser in Windows 10 Mobile, downloaded files are stored in the "Downloads" folder on the device.
You can access this folder using the Files app that is included with W10M. You can then open, copy/move, or delete the file.
You can also use an SD card to store downloads. When a SD card is inserted, you'll be asked whether or not to use it for Pictures, Videos, Music and Downloads. You can control this via Settings -> System -> Storage.