diff --git a/.github/workflows/explainers.yml b/.github/workflows/explainers.yml
new file mode 100644
index 0000000..0f076d8
--- /dev/null
+++ b/.github/workflows/explainers.yml
@@ -0,0 +1,41 @@
+name: Explainers
+
+# Explainers PDFs must be valid, single-page, and correctly sized.
+# This check validates the published contract only; rendering lives
+# in transitrix/coordinator.
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'explainers/**'
+      - 'scripts/check_explainers.py'
+      - '.github/workflows/explainers.yml'
+  pull_request:
+    branches: [main]
+    paths:
+      - 'explainers/**'
+      - 'scripts/check_explainers.py'
+      - '.github/workflows/explainers.yml'
+
+permissions:
+  contents: read
+
+jobs:
+  check-explainers:
+    runs-on: ubuntu-latest
+    name: explainers/ published PDF contract holds
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+
+      - name: Install PDF validator
+        run: python3 -m pip install 'pypdf==6.16.2'
+
+      - name: Check explainers/ contract
+        run: python3 scripts/check_explainers.py
diff --git a/explainers/how-a-document-prints/how-a-document-prints.pdf b/explainers/how-a-document-prints/how-a-document-prints.pdf
new file mode 100644
index 0000000..7fd5b50
Binary files /dev/null and b/explainers/how-a-document-prints/how-a-document-prints.pdf differ
diff --git a/explainers/how-a-document-prints/index.html b/explainers/how-a-document-prints/index.html
index 760b909..6a876d5 100644
--- a/explainers/how-a-document-prints/index.html
+++ b/explainers/how-a-document-prints/index.html
@@ -46,7 +46,7 @@
   </header>
   <main id="main" class="wrap">
     <div class="hero">
-      <p class="pub-date"><a href="../">&larr; Explainers</a></p>
+      <p class="pub-date"><a href="../">&larr; Explainers</a> · <a href="how-a-document-prints.pdf">one-pager</a></p>
       <h1>How a model-backed document prints</h1>
     </div>
 
diff --git a/scripts/check_explainers.py b/scripts/check_explainers.py
new file mode 100644
index 0000000..18832d8
--- /dev/null
+++ b/scripts/check_explainers.py
@@ -0,0 +1,71 @@
+#!/usr/bin/env python3
+"""Check explainers PDF contract.
+
+Published explainer PDFs must:
+- Be valid PDFs
+- Have exactly one page
+- Have geometry 842 × 474 pt (A4 short form, 297 × 167 mm)
+"""
+
+import glob
+import sys
+from pathlib import Path
+
+try:
+    from pypdf import PdfReader
+except ImportError:
+    print("❌ pypdf not installed; skipping checks", file=sys.stderr)
+    sys.exit(0)
+
+
+def check_explainer_pdfs():
+    """Validate all explainer PDFs."""
+    errors = []
+    pdf_files = glob.glob("explainers/*/[!_]*.pdf")
+
+    if not pdf_files:
+        # No explainer PDFs yet; check passes
+        return errors
+
+    for pdf_path in sorted(pdf_files):
+        try:
+            reader = PdfReader(pdf_path)
+
+            # Check page count
+            page_count = len(reader.pages)
+            if page_count != 1:
+                errors.append(
+                    f"❌ {pdf_path}: expected 1 page, got {page_count}"
+                )
+                continue
+
+            # Check geometry on the one page
+            page = reader.pages[0]
+            mediabox = page.mediabox
+            width = float(mediabox.width)
+            height = float(mediabox.height)
+
+            # Expected: 842 × 474 pt (A4 short form)
+            # Allow ±1pt tolerance for rounding
+            expected_w, expected_h = 842, 474
+            if abs(width - expected_w) > 1 or abs(height - expected_h) > 1:
+                errors.append(
+                    f"❌ {pdf_path}: geometry {width:.0f}×{height:.0f} pt, "
+                    f"expected {expected_w}×{expected_h} pt"
+                )
+            else:
+                print(f"✓ {pdf_path}: {width:.0f}×{height:.0f} pt, 1 page")
+
+        except Exception as e:
+            errors.append(f"❌ {pdf_path}: {e}")
+
+    return errors
+
+
+if __name__ == "__main__":
+    errors = check_explainer_pdfs()
+    if errors:
+        for error in errors:
+            print(error, file=sys.stderr)
+        sys.exit(1)
+    sys.exit(0)