The attachment feature in Adobe Acrobat Reader is not working for an online document

I am hoping that someone can help.  I am trying to use Adobe Acrobat Reader for an online purchase and need to attach a file before the purchase is complete, but the attachment feature is not working.  It is not an available option in the left navigation bar and if I try to go under view, tools and then comment it shows up at the top of my screen, but this time is greyed out.  Any help with this would be great.

I have Windows 10 Pro and the most current version of Adobe Acrobat Reader DC.