Handle non-pdf gracefully in viewer (#5004)

# Description of Changes

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Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
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---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

frontend/src/core/components/viewer/LocalEmbedPDF.tsx

@@ -41,6 +41,8 @@ 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';
+import { useTranslation } from 'react-i18next';
 
 interface LocalEmbedPDFProps {
   file?: File | Blob;
@@ -52,6 +54,7 @@ interface LocalEmbedPDFProps {
 }
 
 export function LocalEmbedPDF({ file, url, enableAnnotations = false, onSignatureAdded, signatureApiRef, historyApiRef }: LocalEmbedPDFProps) {
+  const { t } = useTranslation();
   const [pdfUrl, setPdfUrl] = useState<string | null>(null);
   const [, setAnnotations] = useState<Array<{id: string, pageIndex: number, rect: any}>>([]);
 
@@ -171,6 +174,29 @@ 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 : t('viewer.unknownFile');
+    return (
+      <Center h="100%" w="100%">
+        <Stack align="center" gap="md">
+          <div style={{ fontSize: '48px' }}>📄</div>
+          <Text size="lg" fw={600} c="dimmed">
+            {t('viewer.cannotPreviewFile')}
+          </Text>
+          <Text c="dimmed" size="sm" style={{ textAlign: 'center', maxWidth: '400px' }}>
+            {t('viewer.onlyPdfSupported')}
+          </Text>
+          <PrivateContent>
+            <Text c="dimmed" size="xs" style={{ fontFamily: 'monospace' }}>
+              {fileName}
+            </Text>
+          </PrivateContent>
+        </Stack>
+      </Center>
+    );
+  }
+
   if (isLoading || !engine || !pdfUrl) {
     return <ToolLoadingFallback toolName="PDF Engine" />;
   }