保存pdf文档后，使用image-watermark模块在生成的pdf中添加水印。

var watermark = require('image-watermark'); 
watermark.embedWatermark('/path/to/your/generated/pdf', {'text' : 'sample watermark'});

创建pdf后，使用此包为pdf文件添加水印。pdf-watermark

const PDFWatermark = require('pdf-watermark');

      await PDFWatermark({
        pdf_path: "./newsletter.pdf",
        text: "Gentech",  //optional
        image_path: "./everest.png",
      });

另一个在PDF文档中添加文本水印的解决方案是Aspose.PDF Cloud SDK for Node.js，它是一个商业产品，但每月提供150次免费API调用。
目前支持云存储的PDF文件处理：Assume内部存储，Amazon S3，DropBox，Google Drive存储，Google云存储，Windows Azure存储和FTP存储。但是，我们计划添加对处理来自请求主体（流）的PDF文档的支持。
附言：我是Aspose的开发人员传道者。

const { PdfApi } = require("asposepdfcloud");
const { TextStamp }= require("asposepdfcloud/src/models/textStamp");
const { TextState }= require("asposepdfcloud/src/models/textState");
const { HorizontalAlignment }= require("asposepdfcloud/src/models/horizontalAlignment");
const { VerticalAlignment }= require("asposepdfcloud/src/models/verticalAlignment");
const { Rotation }= require("asposepdfcloud/src/models/rotation");

// Get Client Id and Client Secret from https://dashboard.aspose.cloud/
pdfApi = new PdfApi("xxxxx-xxxx-xxxx-xxxx-xxxxxxxxxx", "xxxxxxxxxxxxxxxxxxxxxxx");
var fs = require('fs');

const name = "Test.pdf";
const pageNumber = 1;
const remoteTempFolder = "Temp";
const localTestDataFolder = "C:\\Temp";
const path = remoteTempFolder + "\\" + name;
var data = fs.readFileSync(localTestDataFolder + "\\" + name);

// Upload File
pdfApi.uploadFile(path, data).then((result) => {  
                     console.log("Uploaded File");    
                    }).catch(function(err) {
    // Deal with an error
    console.log(err);
});

// Add Text Stamp
const textState = new TextState();
textState.fontSize = 14;
textState.font = 'Arial';

const stamp = new TextStamp();
stamp.background = true;
stamp.leftMargin = 1;
stamp.rightMargin = 2;
stamp.topMargin = 3;
stamp.bottomMargin = 4;
stamp.horizontalAlignment = HorizontalAlignment.Center;
stamp.verticalAlignment = VerticalAlignment.Center;
stamp.opacity = 1;
stamp.rotate = Rotation.None;
stamp.rotateAngle = 0;
stamp.xIndent = 0;
stamp.yIndent = 0;
stamp.zoom = 1;
stamp.textAlignment = HorizontalAlignment.Center;
stamp.value = "Aspose.PDF Cloud";
stamp.textState = textState;

pdfApi.postPageTextStamps(name, pageNumber,[stamp], null, remoteTempFolder).then((result) => {    
    console.log(result.body.code);                  
}).catch(function(err) {
    // Deal with an error
    console.log(err);
});
//Download file
const localPath = "C:/Temp/textstamp.pdf";

pdfApi.downloadFile(path).then((result) => {    
    console.log(result.response.statusCode);    
    console.log(result.body.byteLength);    
    fs.writeFileSync(localPath, result.body);
    console.log("File Downloaded");    
}).catch(function(err) {
    // Deal with an error
    console.log(err);
});