Make sure you're updated to the latest versions of Acrobat DC on your computer and on your mobile devices.
Turn on the Mobile Link button. You'll find it at the bottom of the Home tab on your desktop/laptop computer. On my iPhone I find it at the upper right corner (icon) of a phone. Same on my iPad.
Also, make sure you're logged into the same Adobe ID. The Adobe ID is what connects your devices to the same collection of PDF files on the Adobe server.
After that the process is usually automatic. You'll see recently opened files in the Recent section of your device.