Hi!
I just tested an interesting setup and it looks like it works!
I just grabbed my 8bitdo adapter and hooked it to my Samsung Tablet via an OTG adapter and paired the Switch Pro Controller onto it! Guess what? It connects and is lag free! Now that can come in handy with emulators!

Requirements:
1. OTG Capable Tablet/Phone: Samsung Galaxy Tab 3 (for example)
2. Switch Pro Controller: https://www.amazon.com/Nintendo-Switch-Pro-Controller/dp/B01NAWKYZ0
3. OTG USB Adapter for your Tablet/Phone: https://www.ebay.com/p/OEM-Samsung-...Connector-OTG-Adapter-Data-Transfer/600094624
4. 8bitdo Adapter: http://www.8bitdo.com/wireless-usb-adapter/

Steps:
1. Hook the 8bitdo Adapter to your Tablet/Phone OTG Adapter
2. Hook the OTG Adapter to your Tablet/Phone
3. Press the little button at the back of the 8bitdo Adapter to enter Sync mode
4. Press the Sync button on the Pro Controller and press L+R
5. Pro Controller get connected
6. Profit

NB: The Controller will be given Xbox Controller layout I think

 

[maruuu07]

Wtf you can just connect it through Bluetooth without any Adapter no need for any of these fancy extra steps why didn't you try that first lol

 

[Zumoly]

Wtf you can just connect it through Bluetooth without any Adapter no need for any of these fancy extra steps why didn't you try that first lol

I think you missed the "lag free part" of my explanation. Besides nobody said there aren't any other ways.
Yes you can connect the Pro controller via bluetooth but it will / might* lag like hell and won't even be recognized as a standard controller.

*Some people claim it doesn't on their device