Quite often people want to obscure or remove critical data in a PDF file. For example they want to send us a PDF file to examine but remove personal or classified data.
One common way to do this is to have a black box drawn over the text which hides the text or image. This works fine for plain viewing but does not actually remove the data.
The problem is that the PDF file consists of a stream of commands. So while the text cannot be seen when the PDF is displayed, it can easily be viewed by examining the text stream.
So if you want to make sure text cannot be seen, you need to physically modify the PDF text data and not just draw over it.
This post is part of our “Understanding the PDF File Format” series. In each article, we discuss a PDF feature, bug, gotcha or tip. If you wish to learn more about PDF, we have 13 years worth of PDF knowledge and tips, so click here to visit our series index!
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