XBlocks need to be enabled on a per course basis. Instructions for installing an XBlock on a server can be found here:
https://github.com/edx/edx-platform/wiki/Installing-a-new-XBlock#install–update-a-xblock

If this is the first XBlock added to the Open edX instance, you also have to enable Advanced Components:
https://github.com/edx/edx-platform/wiki/Installing-a-new-XBlock#allow-all-advanced-components-first-time-only

To enable an XBlock for a given course, first open the course in Studio. On the top menu click on Settings then Advanced Settings. You should then see a text field next to the label Advanced Module List. The name of the XBlock needs to be added as string in a comma separated list between the brackets.

For example, if you are adding the “pdf” XBlock, and the Advanced Module List already contains:

[ "annotation",
  "openassessment"
]

then you should change it to:

[ "annotation",
  "openassessment",
  "pdf"
]

then click on the “Save Changes” button that appears at the bottom of the page.

If you are having trouble finding the name of the XBlock, visit the appropriate Github page, open the setup.py file, and look for the values contained under “entry_points”

https://github.com/MarCnu/pdfXBlock/blob/8c14efb6938d1d2102330e1d3837e9d11cb60fc8/setup.py#L29