Added code to merge two pdfs using javascript (#981)

* Added code to merge two pdfs using javascript

* Update README.md

* Update README.md

* Update merge.js

* Update merge.js

Author: harshita2319

JavaScript/Merge_pdfs/Merged_pdfs/Hello.pdf

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+# Merging two pdf files
+  This script can be used to merge any two pdf files into one using JavaScript. Run merge.js and upload the pdf files which are to be merged and     the ouput will be a single pdf file obtained by merging the uploaded pdfs.
+  
+## Module Required
+* pdf-lib
+pdf-lib npm is a tool to modify exsisting pdfs using Node.js.
+
+## Installation of the above module
+Install the module using<!-- Italics-->*npm install pdf-lib* command.
+The module will be added to <!-- Italics-->*package.json*
+
+## How to merge pdfs
+* Download or clone the repository.
+* Install the requird module (pdf-lib).
+* Run merge.js and add the pdfs which are to be merged.
+* The merged pdf(named final_merged_pdf.pdf) will appear in the same directory.
+
+## Output
+The files I used <!-- Italics-->*hello.pdf* and <!-- Italics-->*world.pdf* have been added to the folder <!-- Italics-->*Merged_pdfs*.
+Here's the ouput:
+
+## Pdf1
+
+![hello](https://user-images.githubusercontent.com/60272753/114947077-191d3a80-9e6a-11eb-9c0a-d76b1a9b4c26.PNG)
+
+
+## Pdf2
+
+![world](https://user-images.githubusercontent.com/60272753/114947110-2508fc80-9e6a-11eb-83b8-47477de381ef.PNG)
+
+
+
+## Merged Pdf
+
+https://user-images.githubusercontent.com/60272753/114947119-29cdb080-9e6a-11eb-9858-ddb2eb41d702.mp4

JavaScript/Merge_pdfs/Merged_pdfs/World.pdf

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+/*adding req PDFDocument module of pdf-lib and file systems lib,
+need to be downloaded in json package before implementing the code*/
+
+const {PDFDocument} = require('pdf-lib');
+const fileSys = require('fs');
+
+
+// to catch and log errors
+mainFun().catch(errors => console.log(errors));
+
+
+//async main_fun is declared
+async function mainFun(){
+
+    // an empty pdf is created using create() function of file systems
+    const emptyPdf = await PDFDocument.create();
+
+
+    /*adding first pdf using readFileSync function of file systems
+    await is used to make it wait while pdf1 is loading*/
+
+    const pdf1 = await PDFDocument.load(fileSys.readFileSync('./Hello.pdf'));
+
+
+     /*adding second pdf
+    await is used to make it wait while pdf2 is loading*/
+    
+    const pdf2 = await PDFDocument.load(fileSys.readFileSync('./World.pdf'));
+    
+
+    /* pages of pdf1 are copied to empty pdf 
+    
+    getPagesIndices() function of file systems is used to get number of pages in pdf1   
+    to use it as range in for loop while traversing*/
+    const pagePdf1 = await emptyPdf.copyPages(pdf1, pdf1.getPageIndices());
+
+
+
+    //for loop is used to traverse pdf within its index to add pages to emptyPdf
+    for (const page of pagePdf1)
+    {
+
+        //pages added using addPge() fun of file systems
+        emptyPdf.addPage(page);
+    }
+
+
+/* pages of pdf2 are copied to empty pdf 
+    
+    getPagesIndices() function of file systems is used to get number of pages in pdf2
+    to use it as range in for loop while traversing*/
+
+    const pagePdf2 = await emptyPdf.copyPages(pdf2, pdf2.getPageIndices());
+
+    
+    //for loop is used to traverse pdf within its index to add pages to emptyPdf
+    for(const page of pagePdf2){
+
+         //pages added using addPge() fun of file systems
+        emptyPdf.addPage(page);
+    }
+
+    
+    /*using writerFileSync(), function of file sys 
+    contents of emptyPdf are wtitten over FinalMergePdf
+
+    save(): to save the pdf, final pdf will appear but won't process otherwise*/
+
+    fileSys.writeFileSync('./FinalMergedPdf.pdf', await emptyPdf.save());
+
+}
+