If you altered a PDF document but it can still be opened normally by the software, here are a few possible reasons:

1. Structure of PDFs

PDF files are designed to be resilient. They contain sections like:

• Header

• Body (objects)

• Cross-reference table (xref)

• Trailer

If your changes didn’t corrupt these core parts—or if the software can recover them—the file will still open.

2. PDFs Can Contain Redundant or Incremental Updates

PDFs support incremental saving, meaning:

• Edits are added on top of the original file.

• Old content might still exist in the file but is overridden when displayed.

• Unless the xref table is corrupted or the trailer is broken, the PDF will remain readable.

3. The Software Has Recovery Features

Most modern PDF readers (like Adobe Acrobat, Foxit, etc.) can repair small errors in the file. So even if your edits introduced minor issues:

• The software auto-recovers.

• You won’t notice the corruption unless it's severe.

4. Your Edits Were Not at a Critical Part

If you altered non-critical parts (like metadata, non-referenced objects, etc.), the PDF will function normally because:

• The viewer doesn't depend on those parts to render the file.

5. PDF Was Not Saved Properly After Editing

If you used a hex editor or manual file modification but didn’t save it properly:

• Your changes may not have affected the real structure.

• The file loads as if nothing happened.

If you're trying to:

• Corrupt a PDF to make it unreadable → you’d need to damage the xref table or trailer.

• Edit a PDF without detection → you need to understand incremental updates and object references.

• Protect a PDF from editing → consider password protection and digital signatures.