GroupDocs.Redaction allows you to set the page-based scope for your redaction of two types:
page range, a given number of pages at certain offset from the beginning or the end of the page; page area (on each page), which is a top-left based rectangle. All filters inherit from RedactionFilter and as an array are set to Filters property of the ReplacementOptions.
You can combine these filters in One set in order to set the scope of redaction to an area on a specific page....can combine these filters in one set in order to set the scope...with GroupDocs.Redaction in one of these guides: Use PDF redaction...