Improve PDF rendering quality and sharpness

Increased the scale for rendering PDF pages from 0.2 to 0.5 in the
`getViewport` method to enhance resolution and sharpness. Although
CSS scales the canvas down, the higher scale ensures better visual
quality.

Enabled high-quality rendering for the canvas context by setting
`ctx.imageSmoothingEnabled` to `true` and `ctx.imageSmoothingQuality`
to `'high'`, resulting in smoother and sharper PDF content rendering.

EnvelopeGenerator.ReceiverUI/wwwroot/js/pdf-viewer.js

@@ -255,12 +255,19 @@ window.pdfViewer = {
             }
 
             const page = await this.pdfDoc.getPage(pageNum);
-            const viewport = page.getViewport({ scale: 0.2 });
+            // Use higher scale for better quality (0.5 instead of 0.2)
+            // CSS will scale it down but maintain sharpness
+            const viewport = page.getViewport({ scale: 0.5 });
             
             canvas.height = viewport.height;
             canvas.width = viewport.width;
 
             const ctx = canvas.getContext('2d');
+            
+            // Enable high-quality rendering
+            ctx.imageSmoothingEnabled = true;
+            ctx.imageSmoothingQuality = 'high';
+            
             const renderContext = {
                 canvasContext: ctx,
                 viewport: viewport