Copying files to an Android tablet from a Windows 10 computer is easy-peasy.
Plug your USB charging cable into your tablet, then plug the other end into a USB port on your computer.
Windows will pop up a dialog box to give you a choice of actions... Select "Open To View Files" or something to that effect.
Windows will display the storage locations on your tablet... will look something like this...
^^^^ That's my wife's Android tablet.
Choose the location where you want to put your files.
Open another file explorer and select the files from your computer that you want to copy to your tablet.
Copy and paste the files from the Windows computer folder to the tablet computer folder.
Make sure you choose file types supported by apps on your tablet... PDF files work for documents, and MP4 files work for videos.
Close the tablet's file explorer window, then click the USB icon on the Windows computer task bar and eject the tablet’s USB connection (if it’s showing; may not be) before disconnecting your tablet.
Good Luck!!!