diff --git a/src/core/parser.js b/src/core/parser.js
index c40f36a8298b8..94b2e7894cbcd 100644
--- a/src/core/parser.js
+++ b/src/core/parser.js
@@ -1163,8 +1163,8 @@ class Lexer {
     const strBuf = this.strBuf;
     strBuf.length = 0;
     let ch = this.currentChar;
-    let isFirstHex = true;
-    let firstDigit, secondDigit;
+    let firstDigit = -1,
+      digit = -1;
     this._hexStringNumWarn = 0;
 
     while (true) {
@@ -1178,26 +1178,25 @@ class Lexer {
         ch = this.nextChar();
         continue;
       } else {
-        if (isFirstHex) {
-          firstDigit = toHexDigit(ch);
-          if (firstDigit === -1) {
-            this._hexStringWarn(ch);
-            ch = this.nextChar();
-            continue;
-          }
+        digit = toHexDigit(ch);
+        if (digit === -1) {
+          this._hexStringWarn(ch);
+        } else if (firstDigit === -1) {
+          firstDigit = digit;
         } else {
-          secondDigit = toHexDigit(ch);
-          if (secondDigit === -1) {
-            this._hexStringWarn(ch);
-            ch = this.nextChar();
-            continue;
-          }
-          strBuf.push(String.fromCharCode((firstDigit << 4) | secondDigit));
+          strBuf.push(String.fromCharCode((firstDigit << 4) | digit));
+          firstDigit = -1;
         }
-        isFirstHex = !isFirstHex;
         ch = this.nextChar();
       }
     }
+
+    // According to the PDF spec, section "7.3.4.3 Hexadecimal Strings":
+    //  "If the final digit of a hexadecimal string is missing—that is, if there
+    //   is an odd number of digits—the final digit shall be assumed to be 0."
+    if (firstDigit !== -1) {
+      strBuf.push(String.fromCharCode(firstDigit << 4));
+    }
     return strBuf.join("");
   }
 
diff --git a/test/pdfs/.gitignore b/test/pdfs/.gitignore
index af276cad181cd..98581ffbafbc0 100644
--- a/test/pdfs/.gitignore
+++ b/test/pdfs/.gitignore
@@ -564,6 +564,7 @@
 !poppler-90-0-fuzzed.pdf
 !issue14415.pdf
 !issue14307.pdf
+!issue18645.pdf
 !issue14497.pdf
 !bug1799927.pdf
 !issue14502.pdf
diff --git a/test/pdfs/issue18645.pdf b/test/pdfs/issue18645.pdf
new file mode 100644
index 0000000000000..deb824088d455
Binary files /dev/null and b/test/pdfs/issue18645.pdf differ
diff --git a/test/test_manifest.json b/test/test_manifest.json
index 82e0024cb4472..57f5a9f7aa011 100644
--- a/test/test_manifest.json
+++ b/test/test_manifest.json
@@ -7986,6 +7986,13 @@
       }
     }
   },
+  {
+    "id": "issue18645",
+    "file": "pdfs/issue18645.pdf",
+    "md5": "ad05b63db4f21f612adb0900093a3e34",
+    "rounds": 1,
+    "type": "eq"
+  },
   {
     "id": "bug857031",
     "file": "pdfs/bug857031.pdf",
diff --git a/test/unit/parser_spec.js b/test/unit/parser_spec.js
index b286dde8602ff..404d729a15a10 100644
--- a/test/unit/parser_spec.js
+++ b/test/unit/parser_spec.js
@@ -201,11 +201,12 @@ describe("parser", function () {
     });
 
     describe("getHexString", function () {
-      it("should not throw exception on bad input", function () {
-        // '7 0 2 15 5 2 2 2 4 3 2 4' should be parsed as '70 21 55 22 24 32'.
+      it("should handle an odd number of digits", function () {
+        // '7 0 2 15 5 2 2 2 4 3 2 4' should be parsed as
+        // '70 21 55 22 24 32 40'.
         const input = new StringStream("<7 0 2 15 5 2 2 2 4 3 2 4>");
         const lexer = new Lexer(input);
-        expect(lexer.getHexString()).toEqual('p!U"$2');
+        expect(lexer.getHexString()).toEqual('p!U"$2@');
       });
     });