We launched a major update last night with some new features.  One of the new features is whiteout / blackout (aka redact).  Whiteout will cover anything on the PDF with white, which you can then type over.

Blackout will cover anything on the PDF with solid black that hides anything underneath.  So, you can see that something has been blacked out but you don’t know what was there.  This is also called “redact” and is what you see the government frequently do when they release previously classified documents.