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We apologize for the inconvenience caused, as per the description above, the Sign option is grayed out when you try to sign a PDF, Is that correct?
Make sure you are logged in Adobe Acrobat Reader DC with your Adobe ID and password. If the issue persists, please uninstall the Acrobat Reader using Acrobat cleaner tool Download Adobe Reader and Acrobat Cleaner Tool - Adobe Labs || Link: http://labs.adobe.com/downloads/acrobatcleaner.html (Assuming you are using Windows machine)
Try signing any other PDF and check.
Hope this helps, and let us know how it goes.
That didn't work for me on MacOS High Sierra and I'm still unable to sign.
Sorry for the inconvenience caused, if you have tried all the steps/suggestions shared above, please enable the root account on your mac and try signing a PDF.
Let us know how it goes.