alt Hacker NewsPDF is less like an image, and more like a web page where elements can be stacked on top of each other. You can visually obscure things by sticking a black rectangle over the top, but anyone who inspects inside the pdf can remove it or see the text in the source.
There would also be a mix of text documents, and image scans. The way to censor each is different.
Perfectly censoring documents, particularly digital ones is actually surprisingly difficult.
> Perfectly censoring documents, particularly digital ones is actually surprisingly difficult.
But the difficult part is easily repeatable once it's figured out, which is why it surprises me that it's not built into Acrobat as a tool already.