diff --git a/crates/cos/src/document.rs b/crates/cos/src/document.rs
index a0b1f56ee..3ac19869b 100644
--- a/crates/cos/src/document.rs
+++ b/crates/cos/src/document.rs
@@ -851,9 +851,10 @@ impl Document {
         self.repair_log = Arc::new(log);
     }
 
-    /// `true` when there are unsaved edits.
+    /// `true` when there are unsaved edits. Removing an `/Encrypt` dictionary written inline in
+    /// the trailer (as MuPDF writes it) changes no object, and is an edit all the same.
     pub fn is_modified(&self) -> bool {
-        !self.overlay.is_empty()
+        !self.overlay.is_empty() || self.encryption_changed
     }
 
     /// Object numbers changed since the document was opened (or last saved).
diff --git a/crates/cos/tests/encryption.rs b/crates/cos/tests/encryption.rs
index af92cf67c..e05161558 100644
--- a/crates/cos/tests/encryption.rs
+++ b/crates/cos/tests/encryption.rs
@@ -336,3 +336,41 @@ fn an_earlier_revision_opens_with_the_documents_own_credentials() {
         assert_eq!(content(&current.open_with_same_credentials(Arc::new(fixture())).unwrap()), SECRET, "{alg:?}");
     }
 }
+
+/// `protect`, with the `/Encrypt` dictionary written inline in the trailer, as MuPDF writes it,
+/// instead of as an indirect object.
+fn protect_inline(alg: Algorithm, user: &str, owner: &str) -> Vec<u8> {
+    let mut doc = Document::open(Arc::new(fixture())).unwrap();
+    doc.set_encryption(&NewEncryption {
+        algorithm: alg,
+        user_password: user,
+        owner_password: owner,
+        permissions: -1,
+        encrypt_metadata: true,
+        seed: [9; 32],
+    })
+    .unwrap();
+    let dict = doc.get(doc.trailer().reference(b"Encrypt").unwrap()).as_dict().cloned().unwrap();
+    doc.trailer_mut().set(b"Encrypt".to_vec(), Object::Dict(dict));
+    write_full(&doc, &SaveOptions::default()).unwrap()
+}
+
+/// MuPDF writes `/Encrypt` inline in the trailer, so removing it changes no object. It is still
+/// an unsaved change, and saving must write a file that opens without a password (#2661).
+#[test]
+fn removing_an_inline_encrypt_dictionary_leaves_an_unencrypted_file() {
+    for alg in ALL {
+        let bytes = Arc::new(protect_inline(alg, "user", "owner"));
+        assert!(matches!(Document::open(bytes.clone()), Err(CosError::NeedsPassword)), "{alg:?}");
+        let mut doc = Document::open_with_password(bytes, Some("owner")).unwrap();
+        assert!(matches!(doc.trailer().get(b"Encrypt"), Some(Object::Dict(_))), "{alg:?}: inline");
+        assert!(!doc.is_modified(), "{alg:?}");
+        doc.remove_encryption();
+        assert!(doc.is_modified(), "{alg:?}: removing security is an unsaved change");
+        let plain = write_incremental(&doc, &SaveOptions::default()).unwrap();
+        let again = Document::open(Arc::new(plain.clone())).unwrap_or_else(|e| panic!("{alg:?}: {e:?}"));
+        assert!(again.security().is_none() && !again.trailer().contains(b"Encrypt"), "{alg:?}");
+        assert_eq!(content(&again), SECRET, "{alg:?}");
+        assert!(hayro_syntax::Pdf::new(plain).is_ok(), "{alg:?}");
+    }
+}
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index ec305b1ca..d7e630808 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -1186,6 +1186,22 @@ fn the_permissions_password_unlocks_an_open_document() {
     assert!(pdfcraft_cos::Document::open(plain).unwrap().security().is_none(), "security removed");
 }
 
+#[test]
+fn remove_security_from_a_file_with_its_encrypt_dictionary_in_the_trailer() {
+    // MuPDF writes /Encrypt inline in the trailer rather than as an object (#2661).
+    let mut doc = pdfcraft_cos::Document::open_with_password(protected("user", "owner", -1), Some("owner")).unwrap();
+    let r = doc.trailer().reference(b"Encrypt").unwrap();
+    let dict = doc.get(r).as_dict().cloned().unwrap();
+    doc.trailer_mut().set(b"Encrypt".to_vec(), pdfcraft_cos::Object::Dict(dict));
+    let bytes = Arc::new(pdfcraft_cos::write_full(&doc, &Default::default()).unwrap());
+    let mut s = Session::new();
+    let id = s.open("m.pdf", None, bytes, Some("owner")).unwrap();
+    s.apply(id, Edit::RemoveProtection).unwrap();
+    assert!(!s.get(id).unwrap().info.encrypted);
+    let plain = s.save_bytes(id).unwrap();
+    assert!(pdfcraft_cos::Document::open(plain).unwrap().security().is_none(), "security removed");
+}
+
 #[test]
 fn unlocking_needs_the_permissions_password_not_the_open_password() {
     let mut s = Session::new();