Feature/v2/viewer tabs (#4646)

# 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)

### 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.

---------

Co-authored-by: Copilot <[email protected]>
Co-authored-by: James Brunton <[email protected]>
Co-authored-by: Claude <[email protected]>

frontend/src/utils/signatureFlattening.ts

@@ -19,6 +19,7 @@ interface SignatureFlatteningOptions {
   selectors: MinimalFileContextSelectors;
   originalFile?: StirlingFile;
   getScrollState: () => { currentPage: number; totalPages: number };
+  activeFileIndex?: number;
 }
 
 export interface SignatureFlatteningResult {
@@ -28,7 +29,7 @@ export interface SignatureFlatteningResult {
 }
 
 export async function flattenSignatures(options: SignatureFlatteningOptions): Promise<SignatureFlatteningResult | null> {
-  const { signatureApiRef, getImageData, exportActions, selectors, originalFile, getScrollState } = options;
+  const { signatureApiRef, getImageData, exportActions, selectors, originalFile, getScrollState, activeFileIndex } = options;
 
   try {
     // Step 1: Extract all annotations from EmbedPDF before export
@@ -104,10 +105,12 @@ export async function flattenSignatures(options: SignatureFlatteningOptions): Pr
       if (!currentFile) {
         const allFileIds = selectors.getAllFileIds();
         if (allFileIds.length > 0) {
-          const fileStub = selectors.getStirlingFileStub(allFileIds[0]);
-          const fileObject = selectors.getFile(allFileIds[0]);
+          // Use activeFileIndex if provided, otherwise default to 0
+          const fileIndex = activeFileIndex !== undefined && activeFileIndex < allFileIds.length ? activeFileIndex : 0;
+          const fileStub = selectors.getStirlingFileStub(allFileIds[fileIndex]);
+          const fileObject = selectors.getFile(allFileIds[fileIndex]);
           if (fileStub && fileObject) {
-            currentFile = createStirlingFile(fileObject, allFileIds[0] as FileId);
+            currentFile = createStirlingFile(fileObject, allFileIds[fileIndex] as FileId);
           }
         }
       }