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Delphi PDF Embedded viewer with PDF.js

Posted on 31st May 2018 in Delphi FMX



When writing business applications you may need to view PDF files in your application. But this requires that the user has to make use of an external PDF viewer such as Adobe or use the OS’s default web-browser.

With PDF.js there is a chance to provide reliable and consistent PDF viewing capabilities in your application. This blog provides the way forward on how to integrate the PDF.js files into your own application.

PDF.js has mostly been developed by Mozilla which provides a JavaScript library that makes it possible to render PDF files in a browser without using a browser plugin. This library does the rendering but isn’t responsible for providing any other functionality to the user like navigation, zoom levels or printing.

Additionally, there’s a complete viewer (implemented using html, CSS and JavaScript) that does the things mentioned above with all the features controlled within the browser. 

As PDF.js uses many technologies provided by modern browsers only and does not work with old browsers like Internet Explorer 8.

PDF.js is licensed under the “Apache License, Version 2.0” that makes it possible to use it in your own application.

There two ways in which one might want to view a PDF-continuous page method or PDF- Page by page method.

So how do you take advantage of the browser in offline state to open PDF files directly within your application?  Read on to find out how to integrate both methods is described.



  • IDE - Delphi 10.2.3 IDE using the INDY third Party components.
  • js JavaScript library.
  • Created in FMX - the code is portable to VCL.


 The Application


Create a Proof of Concept Delphi styled Application that loads a PDF as though it were fully controlled by the Delphi components. The application must be able to load an embedded PDF and open an external PDF.


Two versions are created:-

  • PDF is vertically scrolled with no page breaks from first page to the last page.
  • PDF shows singular page, opened at first page then button controlled to increase or decrease pages.

This is a standard FMX application using the standard toolset. Indy Encoding components are used which is part of the supplied IDE.

Creation steps

The skeleton of the application will not be explained but the technical core will be highlighted.

Scrollable PDF -How it is set up?

An html file is created with the PDF.js functions to load the PDF to a canvas area in the web browser. The html file will call the external PDF.js library and necessary css files.  These files will reside with the other files to be compiled in the Application. All these files are used to ensure that the PDF to view will be opened in the embedded web browser component.

The checkbox when filled will allow an embedded pdf be opened directly else the open button with the checkbox unchecked will allow the user to select a pdf file on your local OS.

Scrollable PDF Functionality

The pdf loads up and the user can scroll the PDF. 

Single Page Button controlled PDF Functionality

The PDF loads up the first page and the user can click on the next button or return button to load a new page. Only one page is viewed at any time.

If you are familiar with JavaScript and Delphi then the following steps will be easy to follow.

Creating the html file

PDF.js library comprises of text_layer_builder.css and text_layer_builder.js. These files are called by the html form.

Create a div-id ‘container’ for the pdf to be directly loaded. Create a canvas-id ‘the-canvas’

for the embedded pdf.


The ‘function externalPDF(Convertedfile,num)’ will load the binary PDF that is in the variable ‘ConvertedFile’ and the page number in the variable num. The function uses the

‘text_layer_builder.js’ to extract each page-number and then processes the ‘ConvertedFile’.


The container Id is used as the position where the pdf must be rendered on the web-browser’s canvas. It loops through each pages and adds each page onto the previous page with ‘container.appendChild(div)’.

Continuous PDF Pages Code -PDFjsindex.htm

Bayseanblog Page:  


PDF Button Control Pages Code -PDFjsindex.htm

The same JavaScript PDF.JS library files are used. Create a canvas-id ‘the-Canvas’ for the pdf to be directly loaded and / or for the embedded pdf to be loaded to. Functions for the changed pages are included.

This ‘function NextPage(Convertedfile)’ will be called from the next Delphi button and the

‘function PriorPage(Convertedfile)’ will be called from the prior Delphi button.

The ‘function externalPDF(Convertedfile,num)’ will load the binary PDF that is in the variable ‘ConvertedFile’ and the page number in the variable num. The function

‘text_layer_builder.js’ extracts the page-number and then processes the ‘ConvertedFile’.

Finally the page is rendered to the Canvas of the browser.

Bayseanblog Page no:  /   Page: 


The Delphi Code

On clicking on the open pdf button with the checkbox ticked loads the embedded PDF file to load else if unchecked will open the open-dialog for a PDF to be loaded. The first step is to open this file into a ‘stringstream’. This is the fastest method without using third party options. This data is loaded into a string variable ‘sendfileinfo’ using the ‘stringstream.datastring’ method. We write a simple try and repeat process until this string variable is fully loaded or else the process falls over. (This seems to be the best option as all other options including threading will fall over. ) Base64 is called to convert the string into a binary string for the JavaScript to read.

procedure TfmPDFview.SpeedButton1Click(Sender: TObject);
    value: Integer;
   StStream: TStringStream;

 webbrowser1.Reload;  //reload to free Ram

 if not checkbox1.ischecked then
      if opendialog1.Execute then   // will not like large files

   edit1.Text:= opendialog1.FileName;
   //StringStream works faster than Filestream  or Bufferedfilestream
   //TStringStream will not open files with a size close to a gigabyte and greater
   // A third party library replacement will be required to perform this
  StStream := TStringStream.Create('');
  sendfileinfo:= StStream.DataString;
  flength:= 0;
  if Length(StStream.Datastring) <> 0 then
   flength1:= flength;
   flength:=  Length(StStream.datastring);
   flength = flength1;
   if length(sendfileinfo) = Length(Ststream.DataString) then;

   if timer1.Enabled then
   timer1.Enabled:= false;


   edit1.Text:= 'Embedded Base64 encoded File ';
   DataJs:= '';
  DataJS:= Embeddedfile ;
   Timer2.Enabled:= true;



The ‘SendFileInfo’ string is base64encoded using Indy ‘TIDEncoderMime’ using the OSDefault textEncoding. The new string ‘ConvertedFile’ is ready to be sent to the web-browser.

procedure TfmPDFview.Base64encode(Sender: Tobject);
  MimeEncoder:= TIdEncoderMime.Create(nil);
Convertedfile:= MimeEncoder.encodestring(Sendfileinfo,IDGlobal.IndyTextEncoding_OSDefault);


A new JavaScript styled string is created that will call the ‘function externalPDF()’ that resides in the html-file. The call to ‘evaluateJavascript’ then processes the JavaScript in the web-browser.

The JavaScript then processes the binary code to render a PDF.


procedure TfmPDFview.LoadBrowser(Sender: TObject);
 js:= 'externalPDF("' +Convertedfile+ '",1);';



Processing the Embedded file requires a different process. The file in this example is coded as string in the Delphi pas unit. It has the already converted to binary code and the full JavaScript function to load in the html file.

function TfmPDFview.Embeddedfile:string;

     'var pdfData = atob("'+
    'JVBERi0xLjcKCjEgMCBvYmogICUgZW50cnkgcG9pbnQKPDwKICAvVHlwZSAvQ2F0YWxvZwog' +



    'Pj4KZW5kb2JqCgozIDAgb2JqCjw8CiAgL1R5cGUgL1BhZ2UKICAvUGFyZW50IDIgMCBSCiAg' +

    'L1Jlc291cmNlcyA8PAogICAgL0ZvbnQgPDwKICAgICAgL0YxIDQgMCBSIAogICAgPj4KICA+' +

    'PgogIC9Db250ZW50cyA1IDAgUgo+PgplbmRvYmoKCjQgMCBvYmoKPDwKICAvVHlwZSAvRm9u' +

    'dAogIC9TdWJ0eXBlIC9UeXBlMQogIC9CYXNlRm9udCAvVGltZXMtUm9tYW4KPj4KZW5kb2Jq' +

    'Cgo1IDAgb2JqICAlIHBhZ2UgY29udGVudAo8PAogIC9MZW5ndGggNDQKPj4Kc3RyZWFtCkJU' +

    'CjcwIDUwIFRECi9GMSAxMiBUZgooSGVsbG8sIHdvcmxkISkgVGoKRVQKZW5kc3RyZWFtCmVu' +





 // 'alert(pdfData);'+

  ' PDFJS.workerSrc = "pdf.worker.js"; '+
  '  PDFJS.getDocument({data: pdfData}).then(function getPageDelphiEmbed(pdf) {  '+
        // Fetch the first page.
      ' pdf.getPage(1).then(function getPageDelphiEmbed(page) {  '+
     // ' document.getElementById("page_num").textContent = 1;    '  +
       ' document.getElementById("page_count").textContent = 1;'+
          '  var scale = 4.8;' +
       ' var viewport = page.getViewport(scale); '+
          // Prepare canvas using PDF page dimensions.
       '   var canvas = document.getElementById("the-canvas"); '+
       '   var context = canvas.getContext("2d");'+
       '   canvas.height = viewport.height;'+
       '   canvas.width = viewport.width;'+
          // Render PDF page into canvas context.
        '   var renderContext = {      '+
         '    canvasContext: context,'+
         '   viewport: viewport'+
        ' }; '+
        ' page.render(renderContext); '+
       ' }); '+
    ' }); ';

   result:= js;
The string data is loaded into the ‘Datajs’ string variable. Once loaded the timer fires to call the ‘eval function’ which will load the data into the html file.
procedure TfmPDFview.Timer2Timer(Sender: TObject);
timer2.Enabled:= false;

Other Options

  • JavaScript Zooming can be added controlled by Delphi buttons.
  • Text Highlighting to copy pages manually or automatically. Will require a call-back function.
  • Set up towards JavaScript OCR processing. Will require call-back function.

 Android and IOS Platforms

Other platforms have not been tested as this is proof of concept and not production level code. Different platforms require different encoding when loading to a ‘stringstream’ and then on to base64encoding.

  • Create('',Tencoding.ansi…);//will require the correct input coding
  • Base64encoding text-encoding format.
  • The open-file dialog will not function for Android and another routine will be required for this.
  • The embedded option should compile in any platform.


Feel free to download the Source code for these two Applications.


Download Code

Happy Coding

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