Normalize /LastModified

claude.md

@@ -26,6 +26,12 @@ Two places record the same volatile information, and both must be handled:
 
 Plus the trailer file identifier `/ID [<...> <...>]`.
 
+A third, per-producer stamp is `/LastModified` in a page or page-piece (`/PieceInfo`) dictionary. It is
+a plain date string like `/ModDate`, so the same `ZeroPdfString` pass covers it. PDFTron writes one
+onto the form XObject it uses for a watermark:
+`/PieceInfo<</PDFTRON<</LastModified(D:20260729134217Z)/Private/Watermark>>>>`. Note the value can
+follow the key with no separating whitespace.
+
 ### Why values are zeroed rather than removed
 
 Zeroing is **length-preserving**, so every offset in the cross-reference table stays valid and no

readme.md

@@ -19,6 +19,7 @@ A PDF records when it was produced and stamps every render with fresh identifier
 
  * The trailer file identifier `/ID [<...> <...>]`
  * The document information dictionary dates `/CreationDate` and `/ModDate`
+ * The page and page-piece dictionary date `/LastModified`, which a producer stamps with a wall-clock time for its own private data (PDFTron writes one onto the form XObject it uses for a watermark)
  * The XMP metadata dates `xmp:CreateDate`, `xmp:ModifyDate`, and `xmp:MetadataDate`
  * The Dublin Core `dc:date`, whether written as direct text content or nested in an `rdf:Seq`/`rdf:li` array
  * The XMP per-generation identifiers `xmpMM:DocumentID`, `xmpMM:InstanceID`, and `xmpMM:OriginalDocumentID`

src/DeterministicPdf/PdfNormalizer.cs

@@ -2,8 +2,9 @@ namespace DeterministicPdf;
 
 /// <summary>
 /// Neutralizes the non-deterministic fields of a PDF (the trailer <c>/ID</c>, the document
-/// information <c>/CreationDate</c> and <c>/ModDate</c>, and the equivalent XMP metadata dates and
-/// identifiers) so that the same source document always produces byte-identical output.
+/// information <c>/CreationDate</c> and <c>/ModDate</c>, the page and page-piece
+/// <c>/LastModified</c>, and the equivalent XMP metadata dates and identifiers) so that the same
+/// source document always produces byte-identical output.
 /// </summary>
 /// <remarks>
 /// Neutralizing the values (dates and identifiers) is done in place and is length-preserving: only the
@@ -59,6 +60,12 @@ static byte[] NormalizeCore(byte[] data)
         ZeroPdfString(data, "/CreationDate"u8, Fill.Digits);
         ZeroPdfString(data, "/ModDate"u8, Fill.Digits);
 
+        // Page and page-piece dictionary modification date. A producer stamps a wall-clock time here
+        // for its own private data, so it changes on every render even when nothing about the document
+        // did (PDFTron writes one onto the form XObject it uses for a watermark:
+        // /PieceInfo<</PDFTRON<</LastModified(D:...)/Private/Watermark>>>>).
+        ZeroPdfString(data, "/LastModified"u8, Fill.Digits);
+
         // Trailer / cross-reference-stream file identifier: /ID [<...> <...>].
         ZeroFileId(data);
 

src/Tests/PdfNormalizerTests.cs

@@ -24,6 +24,27 @@ public async Task NeutralizesVolatileValues()
         await Assert.That(Normalize(input)).IsEqualTo(expected);
     }
 
+    [Test]
+    public async Task NeutralizesPieceInfoLastModified()
+    {
+        // A producer stamps a wall-clock time into the /LastModified of a page-piece dictionary for
+        // its own private data. PDFTron does this on the form XObject it uses for a watermark, so the
+        // value changes on every render even though nothing about the document did. Note the value
+        // follows the key with no separating whitespace.
+        var input = "/PieceInfo<</PDFTRON<</LastModified(D:20260729134217Z)/Private/Watermark>>>>";
+        var expected = "/PieceInfo<</PDFTRON<</LastModified(D:00000000000000Z)/Private/Watermark>>>>";
+        await Assert.That(Normalize(input)).IsEqualTo(expected);
+    }
+
+    [Test]
+    public async Task NeutralizesPageLastModified()
+    {
+        // The page dictionary form: /LastModified is required on a page that carries a /PieceInfo.
+        var input = "/Type /Page /LastModified (D:20240115093000+05'30') /PieceInfo 12 0 R";
+        var expected = "/Type /Page /LastModified (D:00000000000000+00'00') /PieceInfo 12 0 R";
+        await Assert.That(Normalize(input)).IsEqualTo(expected);
+    }
+
     [Test]
     public async Task NeutralizesDublinCoreDate()
     {
@@ -98,9 +119,9 @@ public async Task CollapsesDifferingValuesToTheSameOutput()
     [Test]
     public async Task LeavesLookalikeKeysUntouched()
     {
-        // /IDTree is a name-tree key (not the file identifier), /ModDateStamp is a different name,
-        // and a self-closing date element has no content: none should be altered.
-        var input = "/IDTree [1 2] /ModDateStamp(20240101) <xmp:CreateDate/>2024";
+        // /IDTree is a name-tree key (not the file identifier), /ModDateStamp and /LastModifiedBy are
+        // different names, and a self-closing date element has no content: none should be altered.
+        var input = "/IDTree [1 2] /ModDateStamp(20240101) /LastModifiedBy(2024) <xmp:CreateDate/>2024";
         await Assert.That(Normalize(input)).IsEqualTo(input);
     }