Let's say I use such request to send file to a user in Telegram:
https://api.telegram.org/bot*****/sendDocument?chat_id=0&document=someLongUrl
Then, I will (probably much later) send this file to other users. Will Telegram download this file from remote server every time I send it? If not, how does it determine so quickly that this particular file has already been downloaded, despite the presence of millions of other files on Telegram servers?
If you send (and thus upload) a file through Telegram's Bot API an unique file_id will be created. This is returned by the API and is a valid value for the document parameter on the sendDocument method.
So if you first send your file by URL, then remember the file_id somewhere, you can keep resending that exact file without Telegram downloading it from the original source.