# Check two PDFs for equivalence in C#

> Check another PDF tool's output against its input in .NET: same objects, same decoded content, JPEGs compared by coefficient. MIT licensed.

Available since 1.0.

Find out whether a PDF written by any tool still holds the same document as the original.

## Short answer

`PdfEquivalence.FindDifference` walks both object graphs from the trailer and compares every
dictionary, array and reference, and every stream after decoding. It returns `null` when the two
are equivalent, or a description of the first difference.

```csharp
byte[] original = File.ReadAllBytes("report.pdf");
foreach (string candidate in new[] { "report-optimized.pdf", "report-edited.pdf" })
{
    string? difference = PdfEquivalence.FindDifference(original, File.ReadAllBytes(candidate));
    Console.WriteLine($"{candidate}: {difference ?? "equivalent"}");
}
```

```bash
dotnet add package PdfNetStack
```

In the sample, the optimized copy is equivalent and the copy with a page removed is not:
`trailer/Root/Pages/Kids: array length 3 vs 2`.

## Notes

This is the check the optimizer runs on its own output. Encoding keys such as `/Length`, `/Filter`
and `/DecodeParms` may differ. JPEG images must match coefficient by coefficient; pass
`allowJpegRequantization: true` to accept requantized JPEGs. From 1.1, an overload that also takes
`minImageSsim` accepts images that another tool downsampled or converted to JPEG, when they match
the source resampled to their size; soft masks, masks and stencil images must still match exactly.
The method and its limits are on the
[fidelity](https://www.jadev-corp.com/concepts/fidelity/) page.

## Related

- [Compress PDF](https://www.jadev-corp.com/guides/compress-pdf/)
- [Command line](https://www.jadev-corp.com/cli/): `pdfnetstack verify original.pdf other.pdf`
- API: [PdfEquivalence](https://www.jadev-corp.com/api/pdfnetstack/pdfequivalence/)

Source: https://www.jadev-corp.com/pdfnetstack/recipes/check-pdf-equivalence/