I don't think you know what concurrently or synchronously means in modern programming contexts.
Nonetheless, this library for Java should help, but looks like the example is for overwriting a block of area with a solid color, and you'd have to know the exact coordinates on the page of the block you wish you color in - so that wouldn't be very programmatic if you're looking for a specific line of text to overwrite.
Is it possible to remove all text occurrences contained in a specified area (red color rectangle area) of ??a pdf document? I want to remove what i...
kb.itextpdf.com
Another option might be this (which could be more programmatic by searching for a particular phrase), but I haven't used it:
A Java API for redaction of sensitive and classified information from the documents. You can redact in adobe pdf, redact in excel or in many of different file formats.
docs.groupdocs.com
There's a .NET flavor too