So anyone you are sending to must have AirDrop enabled (which turns on Bluetooth) and depends on which option they have chosen also eg Contacts Only or Everyone. Noticed when you turn off AirDrop you also need to turn off Bluetooth also. I never have my Bluetooth on, only when I need it.
When you turn Bluetooth on on your device, AirDrop is automatically activated. You can send and receive. If you have never used AirDrop, you should make sure you have the settings set the way you want them. I would not have it set for Everyone. If you are in a restaurant or mall, or whatever, everyone with an Apple device connected to that same WiFi, that is AirDrop capable with their Bluetooth/AirDrop activated will be offered whatever you are AirDropping. Set it to Contacts Only.
Then if you are in a meeting or something and want to AirDrop to everyone there, and you are secure that the WiFi is only accessible in that room, no adjoining offices or the hallway, you can do Everyone if everyone in the meeting is not in your contacts list; but be careful that the WiFi is just available in that room, not in the hall or other offices. That document or photo will be offered to every Apple device that is AirDrop capable connected to that WiFi that has their Bluetooth/AirDrop turned on. Might not be a good thing. So, having it set to contacts only is a good rule of thumb.
I have found, contrary to what the other person had said a day or so ago (can't remember the name) Bluetooth is only turned on automatically on the sender's device when the sender starts the Airdrop process. The sender will not be able turn the Bluetooth on on the receiver's device. If Bluetooth/AirDrop is not turned on in the receiving device, that person simply won't be discovered by the sending device and the AirDrop will not happen.
If I am sending a document, photo or whatever and I have my Bluetooth off, when I select the item and select to Airdrop it, my device's Bluetooth will turn on. It is my choice to send. But, I cannot turn on someone else's Bluetooth/AirDrop for them to receive.
The receiver must already have Bluetooth/AirDrop enabled. You both must also be on the same WiFi connection. The sender's device cannot locate the receiver unless the receiver's Bluetooth is in discovery mode (turned on in settings or by the Controls by swiping). The receiver can turn on his/her AirDrop in the control center and then the Bluetooth will also turn on.
There are a couple ways to turn your Bluetooth on, which makes it difficult to explain because someone will post another way to turn it on.
So, in this explanation I am talking about it by swiping up and using those settings. I was using the Settings icon and accessing there, the way it was explained to me. But swiping up is easier.
So, if you have never used AirDrop, swipe up to access the controls. Towards the bottom, select the AirDrop and whatever setting you want, I prefer the Contacts Only, as stated above. When you do that, you will notice the Bluetooth icon at the top of the control center has now been turned on. You can now send and receive AirDrops. You can then swipe up again and turn your Bluetooth off there. It will deactivate your AirDrop capabilities. Touching the Bluetooth icon again will also activate your AirDrop with the settings you had last used. Nobody else can activate your Bluetooth by trying to send you an AirDrop. If your Bluetooth is off, you will not be discovered and you will not receive the AirDrop.
So if you are AirDropping to someone with an Air Drop able device, and you are both using the same WiFi and your device cannot discover the receiving device, ask that person to check to make sure (s)he has Bluetooth turned on in the settings, or control center accessed by swiping up. Your device will not turn on the receiver's Bluetooth. Been trying that all day off and on and my receiving device with Bluetooth turned off cannot receive and the sending device does not turn the receiving device's Bluetooth on.
After you are done, both sender and receiver, if you do not want your Bluetooth on any longer, remember to go to settings or swipe to access controls and turn it off, which will turn off your AirDrop, also.
I hope I said all of this right. Phew!
Please excuse any typos as I have a hard time typing on an iPad.
