diff --git a/news/changelog-1.10.md b/news/changelog-1.10.md
index 529b08cc4e8..de520e2fb93 100644
--- a/news/changelog-1.10.md
+++ b/news/changelog-1.10.md
@@ -92,4 +92,5 @@ All changes included in 1.10:
 - ([#14359](https://github.com/quarto-dev/quarto-cli/issues/14359)): Fix intermediate `.quarto_ipynb` file not being deleted after rendering a `.qmd` with Jupyter engine, causing numbered variants (`_1`, `_2`, ...) to accumulate on disk across renders.
 - ([#14461](https://github.com/quarto-dev/quarto-cli/issues/14461)): Fix `quarto render --to pdf` aborting with `ERROR: Problem running 'fmtutil-sys --all' to rebuild format tree.` when an automatically-installed LaTeX package's post-update format rebuild fails. Format-tree rebuild is now treated as best-effort housekeeping (matching upstream `tinytex` R behavior) — the failure is logged as a warning and the package install completes.
 - ([#14472](https://github.com/quarto-dev/quarto-cli/issues/14472)): Add support for Kotlin in code annotations and YAML cell options. (author: @barendgehrels)
-- ([#14529](https://github.com/quarto-dev/quarto-cli/issues/14529)): Fix bundled Julia engine path leaking into rendered YAML metadata and pandoc log output when running an installed Quarto. The internal subtree-engine filter only matched the source-tree share-path layout (`resources/extension-subtrees/`) and missed installed layouts where the path is `share/extension-subtrees/`.
\ No newline at end of file
+- ([#14529](https://github.com/quarto-dev/quarto-cli/issues/14529)): Fix bundled Julia engine path leaking into rendered YAML metadata and pandoc log output when running an installed Quarto. The internal subtree-engine filter only matched the source-tree share-path layout (`resources/extension-subtrees/`) and missed installed layouts where the path is `share/extension-subtrees/`.
+- ([#14582](https://github.com/quarto-dev/quarto-cli/issues/14582)): Fix format detection for extension formats (e.g. `acm-pdf`) in project preview, manuscript notebooks, MECA bundles, and website format ordering.
\ No newline at end of file
diff --git a/src/config/format.ts b/src/config/format.ts
index b8adee18a2b..48c486d4cb3 100644
--- a/src/config/format.ts
+++ b/src/config/format.ts
@@ -6,6 +6,7 @@
 
 import { kBaseFormat, kPreferHtml } from "../config/constants.ts";
 import { Format, FormatPandoc } from "./types.ts";
+import { parseFormatString } from "../core/pandoc/pandoc-formats.ts";
 
 export function isPdfOutput(format: string): boolean;
 export function isPdfOutput(format: FormatPandoc): boolean;
@@ -97,14 +98,20 @@ export function isJatsOutput(format?: string | FormatPandoc) {
     format = format?.to || "html";
   }
 
-  return [
+  const jatsFormats = [
     "jats",
     "jats_archiving",
     "jats_articleauthoring",
     "jats_publishing",
-  ].find((formatStr) => {
-    return (format as string).startsWith(formatStr);
-  }) !== undefined;
+  ];
+  const formatStr = format as string;
+  if (jatsFormats.some((f) => formatStr.startsWith(f))) return true;
+  try {
+    const base = parseFormatString(formatStr).baseFormat;
+    return jatsFormats.some((f) => base.startsWith(f));
+  } catch {
+    return false;
+  }
 }
 
 export function isPresentationOutput(format: FormatPandoc) {
@@ -123,7 +130,12 @@ export function isRevealjsOutput(format?: string | FormatPandoc) {
     format = format?.to;
   }
   format = format || "html";
-  return format.startsWith("revealjs");
+  if (format.startsWith("revealjs")) return true;
+  try {
+    return parseFormatString(format).baseFormat.startsWith("revealjs");
+  } catch {
+    return false;
+  }
 }
 
 export function isNativeOutput(format: FormatPandoc) {
@@ -146,7 +158,12 @@ function isFormatTo(format: string | FormatPandoc, to: string) {
   const formatStr = typeof format === "string"
     ? format
     : (format?.to || "html");
-  return formatStr.startsWith(to);
+  if (formatStr.startsWith(to)) return true;
+  try {
+    return parseFormatString(formatStr).baseFormat.startsWith(to);
+  } catch {
+    return false;
+  }
 }
 
 export function isMarkdownOutput(
diff --git a/tests/docs/manual/preview/README.md b/tests/docs/manual/preview/README.md
index a18df4debb7..95f4a9dfc3d 100644
--- a/tests/docs/manual/preview/README.md
+++ b/tests/docs/manual/preview/README.md
@@ -216,6 +216,22 @@ format edit is detected on the **first** render request.
 - **Expected:** No preview process restart. Render runs in the same process.
 - **Catches:** Over-eager invalidation triggering 404 when the format is in fact unchanged
 
+## Extension Format PDF Preview (#14582)
+
+### P1: Core functionality
+
+#### T26: Book preview with extension PDF format
+- **Setup**: `extension-pdf-book/` fixture
+- **Steps**: `quarto preview . --to dummy-extension-pdf --no-browser --port 4444`
+- **Expected**: Browse URL contains `web/viewer.html`. PDF.js viewer loads in browser with live-reload.
+- **Catches**: `isFormatTo` startsWith check failing for extension format strings.
+
+#### T27: Book preview with extension HTML format (no regression)
+- **Setup**: Same fixture
+- **Steps**: `quarto preview . --to dummy-extension-html --no-browser --port 4444`
+- **Expected**: Normal HTML preview with live-reload. No PDF.js viewer.
+- **Catches**: Fix doesn't accidentally enable PDF mode for non-PDF extension formats.
+
 ## Test File Templates
 
 **Minimal Python .qmd:**
diff --git a/tests/docs/manual/preview/extension-pdf-book/_extensions/dummy-extension/_extension.yml b/tests/docs/manual/preview/extension-pdf-book/_extensions/dummy-extension/_extension.yml
new file mode 100644
index 00000000000..ef129693881
--- /dev/null
+++ b/tests/docs/manual/preview/extension-pdf-book/_extensions/dummy-extension/_extension.yml
@@ -0,0 +1,5 @@
+name: dummy-extension
+contributes:
+  formats:
+    pdf: default
+    html: default
diff --git a/tests/docs/manual/preview/extension-pdf-book/_quarto.yml b/tests/docs/manual/preview/extension-pdf-book/_quarto.yml
new file mode 100644
index 00000000000..2db614adf43
--- /dev/null
+++ b/tests/docs/manual/preview/extension-pdf-book/_quarto.yml
@@ -0,0 +1,12 @@
+project:
+  type: book
+
+book:
+  title: "Bug reproduction"
+  chapters:
+    - index.qmd
+    - chapter1.qmd
+
+format:
+  dummy-extension-pdf: default
+  dummy-extension-html: default
diff --git a/tests/docs/manual/preview/extension-pdf-book/chapter1.qmd b/tests/docs/manual/preview/extension-pdf-book/chapter1.qmd
new file mode 100644
index 00000000000..a1709fee991
--- /dev/null
+++ b/tests/docs/manual/preview/extension-pdf-book/chapter1.qmd
@@ -0,0 +1,3 @@
+# Introduction
+
+This is chapter 1.
diff --git a/tests/docs/manual/preview/extension-pdf-book/index.qmd b/tests/docs/manual/preview/extension-pdf-book/index.qmd
new file mode 100644
index 00000000000..df3a49db1fb
--- /dev/null
+++ b/tests/docs/manual/preview/extension-pdf-book/index.qmd
@@ -0,0 +1,5 @@
+---
+title: "Preface"
+---
+
+This is the preface.
diff --git a/tests/unit/format-detection.test.ts b/tests/unit/format-detection.test.ts
new file mode 100644
index 00000000000..dd47cfaf87e
--- /dev/null
+++ b/tests/unit/format-detection.test.ts
@@ -0,0 +1,103 @@
+import { unitTest } from "../test.ts";
+import { assert } from "testing/asserts";
+import {
+  isHtmlOutput,
+  isJatsOutput,
+  isPdfOutput,
+  isRevealjsOutput,
+  isTypstOutput,
+} from "../../src/config/format.ts";
+
+unitTest(
+  "isPdfOutput - direct formats",
+  // deno-lint-ignore require-await
+  async () => {
+    assert(isPdfOutput("pdf"));
+    assert(isPdfOutput("beamer"));
+    assert(!isPdfOutput("html"));
+    assert(!isPdfOutput("typst"));
+    assert(!isPdfOutput("docx"));
+  },
+);
+
+unitTest(
+  "isPdfOutput - extension formats",
+  // deno-lint-ignore require-await
+  async () => {
+    assert(isPdfOutput("acm-pdf"));
+    assert(isPdfOutput("dummy-extension-pdf"));
+    assert(isPdfOutput("acm-2023-pdf"));
+    assert(isPdfOutput("journal-beamer"));
+    assert(!isPdfOutput("acm-html"));
+    assert(!isPdfOutput("my-ext-typst"));
+  },
+);
+
+unitTest(
+  "isPdfOutput - modifiers and edge cases",
+  // deno-lint-ignore require-await
+  async () => {
+    assert(!isPdfOutput("default"));
+    assert(!isPdfOutput(""));
+    assert(isPdfOutput("pdf+draft"));
+    assert(isPdfOutput("acm-pdf+draft"));
+  },
+);
+
+unitTest(
+  "isHtmlOutput - extension formats",
+  // deno-lint-ignore require-await
+  async () => {
+    assert(isHtmlOutput("html"));
+    assert(isHtmlOutput("acm-html"));
+    assert(isHtmlOutput("nature-html"));
+    assert(!isHtmlOutput("acm-pdf"));
+  },
+);
+
+unitTest(
+  "isTypstOutput - extension formats",
+  // deno-lint-ignore require-await
+  async () => {
+    assert(isTypstOutput("typst"));
+    assert(isTypstOutput("orange-book-typst"));
+    assert(!isTypstOutput("acm-pdf"));
+  },
+);
+
+unitTest(
+  "isRevealjsOutput - extension formats",
+  // deno-lint-ignore require-await
+  async () => {
+    assert(isRevealjsOutput("revealjs"));
+    assert(isRevealjsOutput("clean-revealjs"));
+    assert(isRevealjsOutput("my-theme-revealjs"));
+    assert(!isRevealjsOutput("html"));
+    assert(!isRevealjsOutput("acm-pdf"));
+  },
+);
+
+unitTest(
+  "isJatsOutput - extension formats",
+  // deno-lint-ignore require-await
+  async () => {
+    assert(isJatsOutput("jats"));
+    assert(isJatsOutput("jats_archiving"));
+    assert(isJatsOutput("plos-jats"));
+    assert(isJatsOutput("my-journal-jats_archiving"));
+    assert(!isJatsOutput("html"));
+    assert(!isJatsOutput("acm-pdf"));
+  },
+);
+
+unitTest(
+  "format detection - malformed inputs",
+  // deno-lint-ignore require-await
+  async () => {
+    assert(!isPdfOutput("---"));
+    assert(!isPdfOutput("123"));
+    assert(isHtmlOutput(""));  // empty defaults to "html" internally
+    assert(!isRevealjsOutput(""));
+    assert(!isJatsOutput(""));
+  },
+);