fix: normalize and flatten annotations

packages/lib/server-only/pdf/normalize-signature-appearances.ts

@@ -0,0 +1,26 @@
+import type { PDFDocument } from 'pdf-lib';
+import { PDFSignature, rectangle } from 'pdf-lib';
+
+export const normalizeSignatureAppearances = (document: PDFDocument) => {
+  const form = document.getForm();
+
+  for (const field of form.getFields()) {
+    if (field instanceof PDFSignature) {
+      field.acroField.getWidgets().forEach((widget) => {
+        widget.ensureAP();
+
+        try {
+          widget.getNormalAppearance();
+        } catch {
+          const { context } = widget.dict;
+
+          const xobj = context.formXObject([rectangle(0, 0, 0, 0)]);
+
+          const streamRef = context.register(xobj);
+
+          widget.setNormalAppearance(streamRef);
+        }
+      });
+    }
+  }
+};