diff --git a/src/core/xref.js b/src/core/xref.js
index 313f18a71d917..75e5d1626d4e1 100644
--- a/src/core/xref.js
+++ b/src/core/xref.js
@@ -680,6 +680,31 @@ class XRef {
     if (this.topDict) {
       return this.topDict;
     }
+
+    // When no trailer dictionary candidate exists, try picking the first
+    // dictionary that contains a /Root entry (fixes issue18986.pdf).
+    if (!trailerDicts.length) {
+      for (const [num, entry] of this.entries.entries()) {
+        if (!entry) {
+          continue;
+        }
+        const ref = Ref.get(num, entry.gen);
+        let obj;
+
+        try {
+          obj = this.fetch(ref);
+        } catch {
+          continue;
+        }
+        if (obj instanceof BaseStream) {
+          obj = obj.dict;
+        }
+        if (obj instanceof Dict && obj.has("Root")) {
+          return obj;
+        }
+      }
+    }
+
     // nothing helps
     throw new InvalidPDFException("Invalid PDF structure.");
   }
diff --git a/test/pdfs/.gitignore b/test/pdfs/.gitignore
index b10be1106d6b6..4bf8b655473e2 100644
--- a/test/pdfs/.gitignore
+++ b/test/pdfs/.gitignore
@@ -156,6 +156,7 @@
 !bug1020858.pdf
 !prefilled_f1040.pdf
 !bug1050040.pdf
+!issue18986.pdf
 !bug1200096.pdf
 !bug1068432.pdf
 !issue12295.pdf
diff --git a/test/pdfs/issue18986.pdf b/test/pdfs/issue18986.pdf
new file mode 100644
index 0000000000000..f23047bf73c0b
Binary files /dev/null and b/test/pdfs/issue18986.pdf differ
diff --git a/test/test_manifest.json b/test/test_manifest.json
index 7cfd0356ee2fc..65cff4331dbd0 100644
--- a/test/test_manifest.json
+++ b/test/test_manifest.json
@@ -4727,6 +4727,13 @@
     "link": false,
     "type": "eq"
   },
+  {
+    "id": "issue18986",
+    "file": "pdfs/issue18986.pdf",
+    "md5": "e147084fabd9677366f6ae3586dd311b",
+    "rounds": 1,
+    "type": "load"
+  },
   {
     "id": "issue6652",
     "file": "pdfs/issue6652.pdf",
diff --git a/test/unit/api_spec.js b/test/unit/api_spec.js
index bb78c9c32b31d..62380c6d3dba6 100644
--- a/test/unit/api_spec.js
+++ b/test/unit/api_spec.js
@@ -623,7 +623,7 @@ describe("api", function () {
         expect(false).toEqual(true);
       } catch (reason) {
         expect(reason instanceof InvalidPDFException).toEqual(true);
-        expect(reason.message).toEqual("Invalid PDF structure.");
+        expect(reason.message).toEqual("Invalid Root reference.");
       }
 
       await loadingTask.destroy();