PDF 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.