diff --git a/src/google/adk/models/anthropic_llm.py b/src/google/adk/models/anthropic_llm.py
index f965a9906d..21ca25cf2b 100644
--- a/src/google/adk/models/anthropic_llm.py
+++ b/src/google/adk/models/anthropic_llm.py
@@ -76,6 +76,15 @@ def _is_image_part(part: types.Part) -> bool:
   )
 
 
+def _is_pdf_part(part: types.Part) -> bool:
+  """Check if the part contains PDF data."""
+  return (
+      part.inline_data
+      and part.inline_data.mime_type
+      and part.inline_data.mime_type == "application/pdf"
+  )
+
+
 def part_to_message_block(
     part: types.Part,
 ) -> Union[
@@ -83,6 +92,7 @@ def part_to_message_block(
     anthropic_types.ImageBlockParam,
     anthropic_types.ToolUseBlockParam,
     anthropic_types.ToolResultBlockParam,
+    anthropic_types.DocumentBlockParam,  # For PDF document blocks
 ]:
   if part.text:
     return anthropic_types.TextBlockParam(text=part.text, type="text")
@@ -134,6 +144,18 @@ def part_to_message_block(
             type="base64", media_type=part.inline_data.mime_type, data=data
         ),
     )
+  elif _is_pdf_part(part):
+    # Handle PDF documents - Anthropic supports PDFs as document blocks
+    # PDFs are encoded as base64 and sent with document type
+    data = base64.b64encode(part.inline_data.data).decode()
+    return anthropic_types.DocumentBlockParam(
+        type="document",
+        source={
+            "type": "base64",
+            "media_type": part.inline_data.mime_type,
+            "data": data,
+        },
+    )
   elif part.executable_code:
     return anthropic_types.TextBlockParam(
         type="text",
diff --git a/tests/unittests/models/test_anthropic_llm.py b/tests/unittests/models/test_anthropic_llm.py
index 13d615bc32..8c93b71bb7 100644
--- a/tests/unittests/models/test_anthropic_llm.py
+++ b/tests/unittests/models/test_anthropic_llm.py
@@ -465,6 +465,39 @@ def test_part_to_message_block_with_multiple_content_items():
   assert result["content"] == "First part\nSecond part"
 
 
+def test_part_to_message_block_with_pdf():
+  """Test that part_to_message_block handles PDF documents."""
+  import base64
+
+  from anthropic import types as anthropic_types
+  from google.adk.models.anthropic_llm import part_to_message_block
+
+  # Create a PDF part with inline data
+  pdf_data = (
+      b"%PDF-1.4\n1 0 obj\n<<\n/Type /Catalog\n>>\nendobj\nxref\n0"
+      b" 1\ntrailer\n<<\n/Root 1 0 R\n>>\n%%EOF"
+  )
+  pdf_part = types.Part(
+      inline_data=types.Blob(
+          mime_type="application/pdf",
+          data=pdf_data,
+      )
+  )
+
+  result = part_to_message_block(pdf_part)
+
+  # PDF should be returned as DocumentBlockParam (TypedDict, which is a dict)
+  assert isinstance(result, dict)
+  # Verify it matches DocumentBlockParam structure
+  assert result["type"] == "document"
+  assert "source" in result
+  assert result["source"]["type"] == "base64"
+  assert result["source"]["media_type"] == "application/pdf"
+  # Verify the data is base64 encoded and can be decoded back
+  decoded_data = base64.b64decode(result["source"]["data"])
+  assert decoded_data == pdf_data
+
+
 content_to_message_param_test_cases = [
     (
         "user_role_with_text_and_image",