Add file stamp

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/esign/stamp.go

@@ -0,0 +1,42 @@
+// Copyright (c) 2025 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package esign
+
+import (
+	"bytes"
+	"fmt"
+
+	"github.com/pdfcpu/pdfcpu/pkg/api"
+	"github.com/pdfcpu/pdfcpu/pkg/pdfcpu/types"
+)
+
+func StampSignatureID(pdfData []byte, signatureID string) ([]byte, error) {
+	text := fmt.Sprintf("Electronic Signature ID: %s", signatureID)
+	desc := "fontname:Helvetica, points:9, pos:tl, rot:0, op:1.0, scale:1.0 abs, color:0.3 0.3 0.3, offset:10 -10"
+
+	wm, err := api.TextWatermark(text, desc, true, false, types.POINTS)
+	if err != nil {
+		return nil, fmt.Errorf("cannot create signature ID stamp: %w", err)
+	}
+
+	reader := bytes.NewReader(pdfData)
+	var buf bytes.Buffer
+
+	if err := api.AddWatermarks(reader, &buf, nil, wm, nil); err != nil {
+		return nil, fmt.Errorf("cannot stamp signature ID on PDF: %w", err)
+	}
+
+	return buf.Bytes(), nil
+}