The discussion has drifted from redacting the SkyMiles number on a PDF receipt to similar redactions on boarding passes. I want to caution you all about two additional facts regarding the PDF downloaded from Delta.
1) The PDF receipt has “hidden text” on page 1. Not sure what (if any) purpose Delta has for embedding this code.
2) The PDF file also contains extensive metadata related to the receipt.
Adobe Acrobat not only lets one redact personally identifying information from the body of the receipt, but it also can detect and remove the hidden text and metadata. Printing, scratching through, and re-scanning should also accomplish those tasks, but is a far more time-consuming and less elegant solution IMO.