Select toolbars and extensions first on the left, then go down to pick all add-ons in the popup menu. Then on the right, you should see Adobe PDF Reader in the list. This should show 32-bit and 64-bit versions there. If one is missing, repair your Adobe Reader installation. Seems like one version of Adobe Reader shipped this year where the 64-bit part was missing. You may get current one where it is included again.
PS: See also Windows updates breaks PDF display in FileMaker