diff --git a/frontend/src/core/components/viewer/LocalEmbedPDF.tsx b/frontend/src/core/components/viewer/LocalEmbedPDF.tsx
index c08eabcbb7..5ba5e3d283 100644
--- a/frontend/src/core/components/viewer/LocalEmbedPDF.tsx
+++ b/frontend/src/core/components/viewer/LocalEmbedPDF.tsx
@@ -41,6 +41,7 @@ import { HistoryAPIBridge } from '@app/components/viewer/HistoryAPIBridge';
 import type { SignatureAPI, HistoryAPI } from '@app/components/viewer/viewerTypes';
 import { ExportAPIBridge } from '@app/components/viewer/ExportAPIBridge';
 import { BookmarkAPIBridge } from '@app/components/viewer/BookmarkAPIBridge';
+import { isPdfFile } from '@app/utils/fileUtils';
 
 interface LocalEmbedPDFProps {
   file?: File | Blob;
@@ -171,6 +172,27 @@ export function LocalEmbedPDF({ file, url, enableAnnotations = false, onSignatur
     );
   }
 
+  // Check if the file is actually a PDF
+  if (file && !isPdfFile(file)) {
+    const fileName = 'name' in file ? file.name : 'Unknown file';
+    return (
+      <Center h="100%" w="100%">
+        <Stack align="center" gap="md">
+          <div style={{ fontSize: '48px' }}>📄</div>
+          <Text size="lg" fw={600} c="dimmed">
+            Cannot Preview File
+          </Text>
+          <Text c="dimmed" size="sm" style={{ textAlign: 'center', maxWidth: '400px' }}>
+            The viewer only supports PDF files. This file appears to be a different format.
+          </Text>
+          <Text c="dimmed" size="xs" style={{ fontFamily: 'monospace' }}>
+            {fileName}
+          </Text>
+        </Stack>
+      </Center>
+    );
+  }
+
   if (isLoading || !engine || !pdfUrl) {
     return <ToolLoadingFallback toolName="PDF Engine" />;
   }
diff --git a/frontend/src/core/utils/fileUtils.test.ts b/frontend/src/core/utils/fileUtils.test.ts
new file mode 100644
index 0000000000..b833c5f6d5
--- /dev/null
+++ b/frontend/src/core/utils/fileUtils.test.ts
@@ -0,0 +1,87 @@
+import { describe, it, expect } from 'vitest';
+import { isPdfFile, detectFileExtension, formatFileSize } from './fileUtils';
+
+describe('fileUtils', () => {
+  describe('isPdfFile', () => {
+    it('should return true for PDF files with correct MIME type', () => {
+      const pdfFile = new File(['content'], 'document.pdf', { type: 'application/pdf' });
+      expect(isPdfFile(pdfFile)).toBe(true);
+    });
+
+    it('should return true for PDF files with .pdf extension even without MIME type', () => {
+      const pdfFile = new File(['content'], 'document.pdf', { type: '' });
+      expect(isPdfFile(pdfFile)).toBe(true);
+    });
+
+    it('should return false for non-PDF files', () => {
+      const txtFile = new File(['content'], 'document.txt', { type: 'text/plain' });
+      expect(isPdfFile(txtFile)).toBe(false);
+    });
+
+    it('should return false for image files', () => {
+      const imageFile = new File(['content'], 'image.png', { type: 'image/png' });
+      expect(isPdfFile(imageFile)).toBe(false);
+    });
+
+    it('should return false for null', () => {
+      expect(isPdfFile(null)).toBe(false);
+    });
+
+    it('should return false for undefined', () => {
+      expect(isPdfFile(undefined)).toBe(false);
+    });
+
+    it('should handle file-like objects with name and type', () => {
+      const fileLike = { name: 'test.pdf', type: 'application/pdf' };
+      expect(isPdfFile(fileLike)).toBe(true);
+    });
+
+    it('should handle file-like objects with PDF extension but no type', () => {
+      const fileLike = { name: 'test.pdf', type: '' };
+      expect(isPdfFile(fileLike)).toBe(true);
+    });
+  });
+
+  describe('detectFileExtension', () => {
+    it('should detect PDF extension', () => {
+      expect(detectFileExtension('document.pdf')).toBe('pdf');
+    });
+
+    it('should detect extension in uppercase', () => {
+      expect(detectFileExtension('document.PDF')).toBe('pdf');
+    });
+
+    it('should return empty string for files without extension', () => {
+      expect(detectFileExtension('document')).toBe('');
+    });
+
+    it('should handle multiple dots in filename', () => {
+      expect(detectFileExtension('my.document.pdf')).toBe('pdf');
+    });
+
+    it('should normalize jpeg to jpg', () => {
+      expect(detectFileExtension('image.jpeg')).toBe('jpg');
+    });
+  });
+
+  describe('formatFileSize', () => {
+    it('should format bytes', () => {
+      expect(formatFileSize(0)).toBe('0 B');
+      expect(formatFileSize(500)).toBe('500 B');
+    });
+
+    it('should format kilobytes', () => {
+      expect(formatFileSize(1024)).toBe('1 KB');
+      expect(formatFileSize(2048)).toBe('2 KB');
+    });
+
+    it('should format megabytes', () => {
+      expect(formatFileSize(1024 * 1024)).toBe('1 MB');
+      expect(formatFileSize(5 * 1024 * 1024)).toBe('5 MB');
+    });
+
+    it('should format gigabytes', () => {
+      expect(formatFileSize(1024 * 1024 * 1024)).toBe('1 GB');
+    });
+  });
+});
diff --git a/frontend/src/core/utils/fileUtils.ts b/frontend/src/core/utils/fileUtils.ts
index b7e3a429ca..0f14714019 100644
--- a/frontend/src/core/utils/fileUtils.ts
+++ b/frontend/src/core/utils/fileUtils.ts
@@ -51,3 +51,26 @@ export function detectFileExtension(filename: string): string {
 
   return extension;
 }
+
+/**
+ * Checks if a file is a PDF based on extension and MIME type
+ * @param file - File or file-like object with name and type properties
+ * @returns true if the file appears to be a PDF
+ */
+export function isPdfFile(file: { name?: string; type?: string } | File | Blob | null | undefined): boolean {
+  if (!file) return false;
+
+  const name = 'name' in file ? file.name : undefined;
+  const type = file.type;
+
+  // Check MIME type first (most reliable)
+  if (type === 'application/pdf') return true;
+
+  // Check file extension as fallback
+  if (name) {
+    const ext = detectFileExtension(name);
+    if (ext === 'pdf') return true;
+  }
+
+  return false;
+}