diff --git a/app/window.py b/app/window.py
index 4693ce4..715e209 100644
--- a/app/window.py
+++ b/app/window.py
@@ -237,7 +237,7 @@ def _a11y(btn, tip):
         self._help_btn.setObjectName("theme_btn")
         _a11y(self._help_btn, t("help.tip"))
         self._help_btn.setFixedSize(28, 28)
-        self._help_btn.clicked.connect(lambda: __import__('webbrowser').open("https://nelsonduarte.github.io/PDFApps/#guide"))
+        self._help_btn.clicked.connect(lambda: __import__('webbrowser').open("https://pdf-apps.com/docs#first-steps"))
         wb_h.addWidget(self._help_btn)
 
         _lang_labels = {"en": "EN", "pt": "PT", "es": "ES", "fr": "FR", "de": "DE", "zh": "ZH", "it": "IT", "nl": "NL"}
diff --git a/tests/test_help_url_domain.py b/tests/test_help_url_domain.py
new file mode 100644
index 0000000..2df3ad5
--- /dev/null
+++ b/tests/test_help_url_domain.py
@@ -0,0 +1,116 @@
+"""Guards against user-facing URLs in app/ pointing at a retired domain.
+
+History of the bug this test exists for (verified with ``git log -S``):
+
+* ``b14d11d`` (2026-03-25) added the Help ("?") button and, in the *same*
+  commit, the ``<section id="guide">`` plus its ``<li><a href="#guide">``
+  nav entry in ``docs/index.html``. The target the button pointed at was
+  created alongside it, so the link worked.
+* ``6847858`` (2026-03-27) rewrote the URL's repo path from ``PDFApps-en``
+  to ``PDFApps``. That is the form that shipped in 1.15.0. (The original
+  ``/PDFApps-en/`` path is a 404 today, so only this commit made the URL
+  point at a live page.)
+* ``350a419`` (2026-04-08) replaced the single-page site with the five-page
+  layout and, in doing so, deleted both the ``#guide`` section and its nav
+  link. Nothing has re-added them since.
+
+So the Help button worked for about two weeks and broke in the *website
+redesign*, not in the later migration to Cloudflare Pages. The migration
+changed the canonical host to ``pdf-apps.com`` but was not the cause; do
+not re-attribute this to Cloudflare.
+
+Why the breakage is silent, and therefore why this test exists: the old
+``nelsonduarte.github.io/PDFApps/`` host still answers **200**. It does not
+404, and it is not a stale snapshot either: it now serves the current
+five-page content, whose navigation no longer contains a ``#guide`` anchor.
+An unknown fragment is not an error in HTTP or in any browser, so the user
+silently landed at the top of a page that no longer had the section, with
+nothing anywhere to signal the link was dead. No status code, no log, no
+crash can catch this class of bug; only an assertion like the ones below.
+
+These tests are deliberately scoped to the *host*, not to the full URL:
+asserting the exact string would turn any legitimate anchor change (e.g.
+``#first-steps`` -> ``#install``) into a red test, which would block a valid
+edit. Matching ``github.io`` catches the whole class of stale-domain
+regressions in one assertion and survives anchor churn.
+"""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parent.parent
+APP = ROOT / "app"
+
+# The retired documentation/website host. Note this is intentionally narrow:
+# api.github.com and github.com/... links are live and legitimate (updater,
+# OCR help), so only the Pages host is forbidden.
+DEAD_HOST = "github.io"
+
+CANONICAL_HOST = "pdf-apps.com"
+
+
+def _python_sources() -> list[Path]:
+    return sorted(p for p in APP.rglob("*.py") if "__pycache__" not in p.parts)
+
+
+def test_no_app_source_references_the_retired_pages_domain():
+    """No Python source under app/ may link to the old GitHub Pages host."""
+    offenders: list[str] = []
+    for path in _python_sources():
+        for lineno, line in enumerate(
+            path.read_text(encoding="utf-8").splitlines(), start=1
+        ):
+            if DEAD_HOST in line:
+                offenders.append(f"{path.relative_to(ROOT)}:{lineno}: {line.strip()}")
+
+    assert not offenders, (
+        "app/ must not reference the retired "
+        f"{DEAD_HOST!r} site (canonical host is {CANONICAL_HOST}).\n"
+        + "\n".join(offenders)
+    )
+
+
+def _json_data_files() -> list[Path]:
+    return sorted(p for p in APP.rglob("*.json") if "__pycache__" not in p.parts)
+
+
+def test_no_json_data_file_references_the_retired_pages_domain():
+    """Shipped JSON data under app/ must not carry the dead host either.
+
+    Scoped to every ``*.json`` under app/ rather than to translations.json
+    alone: today that glob resolves to exactly one file, so this does not
+    change what is currently checked, but it makes the guard cover any JSON
+    data file added later instead of silently exempting it. translations.json
+    remains the motivating case, being the one place a help link could be
+    reintroduced in 8 languages at once.
+    """
+    offenders: list[str] = []
+    for path in _json_data_files():
+        if DEAD_HOST in path.read_text(encoding="utf-8"):
+            offenders.append(str(path.relative_to(ROOT)))
+
+    assert not offenders, (
+        f"JSON data under app/ must not reference {DEAD_HOST!r} "
+        f"(canonical host is {CANONICAL_HOST}): " + ", ".join(offenders)
+    )
+
+
+def test_help_button_opens_the_canonical_site():
+    """The Help button's URL must live on the canonical host.
+
+    Pinned to the host and the docs path only, so the section anchor stays
+    free to change without breaking this test.
+    """
+    src = (APP / "window.py").read_text(encoding="utf-8")
+    match = re.search(r'_help_btn\.clicked\.connect\([^\n]*?open\("([^"]+)"\)', src)
+    assert match, "Could not locate the Help button's clicked->open(...) wiring."
+
+    url = match.group(1)
+    assert url.startswith(f"https://{CANONICAL_HOST}/"), (
+        f"Help URL {url!r} must point at https://{CANONICAL_HOST}/"
+    )
+    assert "/docs" in url, (
+        f"Help URL {url!r} should open the documentation page, not the landing page."
+    )