Adobe Acrobat Pro DC 2016 question regarding "OCR Text Recognition"

You can now open multiple documents in a single instance of the Acrobat software. Each document displays in it's own tab next to the Home and Tools tabs.  To switch between documents, you simply click on that document's tab.  Hence the name "Document Tab."

So the instruction to click on the document tab is meant to make sure you have the correct document selected before you begin. 

The previous examples above don't have multiple document tabs, so your actions will apply to the single open document.  But if you have multiple document tabs displayed (i.e., multiple open documents), you want to make sure you start from the document to be converted not a different document.