If you share documents often, then you will likely know the word “Redaction.”   Colloquially, redaction is blacking out  sensitive information on a document.  Adobe Acrobat Pro has a redaction tool built in, found under the “Advanced” tab, which makes it easy to select and highlight the data you want to hide; however,   there is a well known and compromising glitch.  Even if the data is blacked out via redaction, if the document is merely saved, the hidden data could be revealed by copying and pasting said data into a new document. Not so bueno!

If redaction is a daily part of your work regimen, then you are probably utilizing a tool like Redax to properly redact data.  However, if you only utilize this function on occasion,  then know that you must first save the document as a TIFF – turning the document into an image ensures the  blacked out data cannot be retrieved.  Next, save the TIFF version  as a PDF. Your blacked out content will finally be embedded in the PDF.

For more information, here is an informative, albeit verbose article:

http://www.acrobatusers.com/tutorials/redacting-pdf-files-survey-tools