diff --git a/EnvelopeGenerator.ReceiverUI/Pages/EnvelopeViewer.razor b/EnvelopeGenerator.ReceiverUI/Pages/EnvelopeViewer.razor
index 8253d971..9efca85d 100644
--- a/EnvelopeGenerator.ReceiverUI/Pages/EnvelopeViewer.razor
+++ b/EnvelopeGenerator.ReceiverUI/Pages/EnvelopeViewer.razor
@@ -130,7 +130,7 @@
                 <div class="pdf-frame">
                     @if (_pdfLoaded && _showThumbnails) {
                         <!-- PDF Thumbnail Sidebar -->
-                        <div class="pdf-thumbnails">
+                        <div class="pdf-thumbnails" style="width: @(_thumbnailWidth)px">
                             <div class="pdf-thumbnails__content">
                                 @for (int i = 1; i <= _totalPages; i++) {
                                     var pageNum = i;
@@ -143,6 +143,11 @@
                                 }
                             </div>
                         </div>
+                        <!-- Resizable Splitter -->
+                        <div class="pdf-splitter @(_isResizing ? "resizing" : "")" 
+                             @onmousedown="OnSplitterMouseDown"
+                             @onmousedown:preventDefault="true">
+                        </div>
                     }
                     <div class="pdf-canvas-wrapper">
                         <canvas id="pdf-canvas" class="pdf-canvas"></canvas>
@@ -165,19 +170,27 @@
 </div>
 
 @code {
-    [Parameter] public string? EnvelopeKey { get; set; }
+[Parameter] public string? EnvelopeKey { get; set; }
 
-    bool _isLoading = true;
-    string? _errorMessage;
-    string? _pdfDataUrl;
-    bool _pdfLoaded = false;
-    int _currentPage = 1;
-    int _totalPages = 0;
-    int _currentZoom = 150;
-    bool _showThumbnails = true;
-    DotNetObjectReference<EnvelopeViewer>? _dotNetRef;
+bool _isLoading = true;
+string? _errorMessage;
+string? _pdfDataUrl;
+bool _pdfLoaded = false;
+int _currentPage = 1;
+int _totalPages = 0;
+int _currentZoom = 150;
+bool _showThumbnails = true;
+DotNetObjectReference<EnvelopeViewer>? _dotNetRef;
 
-    protected override async Task OnInitializedAsync() {
+// Resizable splitter state
+int _thumbnailWidth = 260;
+bool _isResizing = false;
+int _resizeStartX = 0;
+int _resizeStartWidth = 0;
+const int MinThumbnailWidth = 150;
+const int MaxThumbnailWidth = 400;
+
+protected override async Task OnInitializedAsync() {
         if (string.IsNullOrWhiteSpace(EnvelopeKey)) {
             _errorMessage = "Envelope-Schlüssel fehlt.";
             _isLoading = false;
@@ -205,6 +218,19 @@
     }
 
     protected override async Task OnAfterRenderAsync(bool firstRender) {
+        if (firstRender) {
+            // Load saved thumbnail width from localStorage
+            try {
+                var savedWidth = await JSRuntime.InvokeAsync<string>("localStorage.getItem", "envelopeViewer_thumbnailWidth");
+                if (!string.IsNullOrEmpty(savedWidth) && int.TryParse(savedWidth, out var width)) {
+                    _thumbnailWidth = Math.Clamp(width, MinThumbnailWidth, MaxThumbnailWidth);
+                    await InvokeAsync(StateHasChanged);
+                }
+            } catch {
+                // Ignore localStorage errors
+            }
+        }
+
         if (!_pdfLoaded && !string.IsNullOrWhiteSpace(_pdfDataUrl)) {
             await Task.Delay(500);
             
@@ -216,6 +242,10 @@
                     _pdfLoaded = true;
                     _totalPages = await JSRuntime.InvokeAsync<int>("pdfViewer.getTotalPages");
                     _currentPage = await JSRuntime.InvokeAsync<int>("pdfViewer.getCurrentPage");
+                    
+                    // Attach resize listeners
+                    await JSRuntime.InvokeVoidAsync("pdfViewer.attachResizeListeners", _dotNetRef);
+                    
                     await InvokeAsync(StateHasChanged);
                     
                     // Wait for DOM to be ready, then render thumbnails
@@ -315,6 +345,45 @@
         }
     }
 
+    // Resizable splitter methods
+    void OnSplitterMouseDown(MouseEventArgs e) {
+        _isResizing = true;
+        _resizeStartX = (int)e.ClientX;
+        _resizeStartWidth = _thumbnailWidth;
+        
+        // Add resizing class to body to prevent text selection
+        _ = JSRuntime.InvokeVoidAsync("eval", "document.body.classList.add('resizing')");
+        _ = JSRuntime.InvokeVoidAsync("pdfViewer.startResize");
+    }
+
+    [JSInvokable]
+    public async Task OnSplitterMouseMove(int clientX) {
+        if (!_isResizing) return;
+
+        var delta = clientX - _resizeStartX;
+        var newWidth = _resizeStartWidth + delta;
+        
+        // Clamp to min/max
+        _thumbnailWidth = Math.Clamp(newWidth, MinThumbnailWidth, MaxThumbnailWidth);
+        
+        await InvokeAsync(StateHasChanged);
+    }
+
+    [JSInvokable]
+    public async Task OnSplitterMouseUp() {
+        if (!_isResizing) return;
+        
+        _isResizing = false;
+        
+        // Remove resizing class from body
+        await JSRuntime.InvokeVoidAsync("eval", "document.body.classList.remove('resizing')");
+        
+        // Save preference to localStorage
+        await JSRuntime.InvokeVoidAsync("localStorage.setItem", "envelopeViewer_thumbnailWidth", _thumbnailWidth.ToString());
+        
+        await InvokeAsync(StateHasChanged);
+    }
+
     public async ValueTask DisposeAsync() {
         if (_pdfLoaded) {
             try {
diff --git a/EnvelopeGenerator.ReceiverUI/wwwroot/css/envelope-viewer.css b/EnvelopeGenerator.ReceiverUI/wwwroot/css/envelope-viewer.css
index 1b532cfb..82179019 100644
--- a/EnvelopeGenerator.ReceiverUI/wwwroot/css/envelope-viewer.css
+++ b/EnvelopeGenerator.ReceiverUI/wwwroot/css/envelope-viewer.css
@@ -104,6 +104,46 @@
     background: linear-gradient(135deg, #6b1cb0 0%, #1e3a72 100%);
 }
 
+.pdf-splitter {
+    width: 4px;
+    background: transparent;
+    cursor: col-resize;
+    flex-shrink: 0;
+    position: relative;
+    transition: background 0.2s ease;
+    z-index: 10;
+    user-select: none;
+}
+
+.pdf-splitter::before {
+    content: '';
+    position: absolute;
+    left: -4px;
+    right: -4px;
+    top: 0;
+    bottom: 0;
+    /* Enlarged hitbox for easier grabbing */
+}
+
+.pdf-splitter:hover,
+.pdf-splitter.resizing {
+    background: linear-gradient(90deg, 
+        rgba(126, 34, 206, 0.4) 0%, 
+        rgba(42, 82, 152, 0.4) 100%);
+}
+
+.pdf-splitter:active {
+    background: linear-gradient(90deg, 
+        rgba(126, 34, 206, 0.6) 0%, 
+        rgba(42, 82, 152, 0.6) 100%);
+}
+
+/* Prevent text selection during resize */
+body.resizing {
+    user-select: none;
+    cursor: col-resize !important;
+}
+
 .pdf-thumbnail {
     cursor: pointer;
     border-radius: 8px;
diff --git a/EnvelopeGenerator.ReceiverUI/wwwroot/js/pdf-viewer.js b/EnvelopeGenerator.ReceiverUI/wwwroot/js/pdf-viewer.js
index 91c8f650..815a66e2 100644
--- a/EnvelopeGenerator.ReceiverUI/wwwroot/js/pdf-viewer.js
+++ b/EnvelopeGenerator.ReceiverUI/wwwroot/js/pdf-viewer.js
@@ -289,6 +289,48 @@ window.pdfViewer = {
             this.wheelEventAttached = false;
             this.dotNetReference = null;
         }
+        this.detachResizeListeners();
+    },
+
+    // Resizable splitter functionality
+    isResizing: false,
+    resizeMouseMoveHandler: null,
+    resizeMouseUpHandler: null,
+
+    attachResizeListeners(dotNetRef) {
+        this.dotNetReference = dotNetRef;
+
+        this.resizeMouseMoveHandler = (e) => {
+            if (this.isResizing && this.dotNetReference) {
+                this.dotNetReference.invokeMethodAsync('OnSplitterMouseMove', e.clientX);
+            }
+        };
+
+        this.resizeMouseUpHandler = () => {
+            if (this.isResizing && this.dotNetReference) {
+                this.isResizing = false;
+                this.dotNetReference.invokeMethodAsync('OnSplitterMouseUp');
+            }
+        };
+
+        document.addEventListener('mousemove', this.resizeMouseMoveHandler);
+        document.addEventListener('mouseup', this.resizeMouseUpHandler);
+    },
+
+    detachResizeListeners() {
+        if (this.resizeMouseMoveHandler) {
+            document.removeEventListener('mousemove', this.resizeMouseMoveHandler);
+            this.resizeMouseMoveHandler = null;
+        }
+        if (this.resizeMouseUpHandler) {
+            document.removeEventListener('mouseup', this.resizeMouseUpHandler);
+            this.resizeMouseUpHandler = null;
+        }
+        this.isResizing = false;
+    },
+
+    startResize() {
+        this.isResizing = true;
     }
 };