Introduce Run.IsHidden property or similar, which hides the text in the document. In MS Word, text is hidden using the Home -> Font -> Font -> Effects -> Hidden. In the Rtf format, such text is preceded (marked) with a '\v' tag. In OOXML, the tag is <vanish/>. The display: none; style from HTML is mapped to hidden text when document is converted to DOCX. Hidden text is visualized with dotted underline when formatting symbols are shown. Hidden text is not exported to PDF.