How to Merge PDFs — Combine Multiple PDF Files Into One Online or in Code

import { PDFDocument } from 'pdf-lib';
import fs from 'fs';
 
async function mergePdfs(inputPaths, outputPath) {
  const mergedPdf = await PDFDocument.create();
  
  for (const pdfPath of inputPaths) {
    const pdfBytes = fs.readFileSync(pdfPath);
    const pdf = await PDFDocument.load(pdfBytes);
    const copiedPages = await mergedPdf.copyPages(pdf, pdf.getPageIndices());
    copiedPages.forEach(page => mergedPdf.addPage(page));
  }
  
  const mergedBytes = await mergedPdf.save();
  fs.writeFileSync(outputPath, mergedBytes);
  console.log(`Merged ${inputPaths.length} PDFs → ${outputPath}`);
}
 
// Usage
await mergePdfs([
  'chapter-01.pdf',
  'chapter-02.pdf',
  'chapter-03.pdf',
], 'merged-report.pdf');

async function mergePdfsFromFiles(fileList) {
  const mergedPdf = await PDFDocument.create();
  
  for (const file of fileList) {
    const arrayBuffer = await file.arrayBuffer();
    const pdf = await PDFDocument.load(arrayBuffer);
    const copiedPages = await mergedPdf.copyPages(pdf, pdf.getPageIndices());
    copiedPages.forEach(page => mergedPdf.addPage(page));
  }
  
  const mergedBytes = await mergedPdf.save();
  
  // Trigger download
  const blob = new Blob([mergedBytes], { type: 'application/pdf' });
  const url = URL.createObjectURL(blob);
  const a = document.createElement('a');
  a.href = url;
  a.download = 'merged.pdf';
  a.click();
  URL.revokeObjectURL(url);
}

from pypdf import PdfReader, PdfWriter
from pathlib import Path
 
def merge_pdfs(input_paths: list[str], output_path: str) -> dict:
    writer = PdfWriter()
    total_pages = 0
    
    for path in input_paths:
        reader = PdfReader(path)
        pages_added = 0
        for page in reader.pages:
            writer.add_page(page)
            pages_added += 1
        total_pages += pages_added
        print(f"  Added {pages_added} pages from {Path(path).name}")
    
    with open(output_path, 'wb') as f:
        writer.write(f)
    
    return {'files': len(input_paths), 'total_pages': total_pages}
 
# Usage
result = merge_pdfs([
    'chapter-01.pdf',
    'chapter-02.pdf',
    'chapter-03.pdf',
], 'merged-report.pdf')
print(f"Merged {result['files']} files, {result['total_pages']} pages total")

# Merge multiple PDFs into one
gs -dBATCH -dNOPAUSE -dSAFER \
   -sDEVICE=pdfwrite \
   -dCompatibilityLevel=1.7 \
   -sOutputFile=merged.pdf \
   chapter-01.pdf chapter-02.pdf chapter-03.pdf
 
# Merge all PDFs in a directory (alphabetical order)
gs -dBATCH -dNOPAUSE -dSAFER \
   -sDEVICE=pdfwrite \
   -sOutputFile=merged.pdf \
   *.pdf

# Simple merge
pdftk chapter-01.pdf chapter-02.pdf chapter-03.pdf cat output merged.pdf
 
# Merge with specific page ranges from each file
pdftk A=doc1.pdf B=doc2.pdf cat A1-10 B5-end output merged.pdf

from pypdf import PdfReader, PdfWriter
 
def merge_with_bookmarks(input_paths: list[str], output_path: str):
    writer = PdfWriter()
    page_offset = 0
    
    for path in input_paths:
        reader = PdfReader(path)
        filename = Path(path).stem
        pages_in_file = len(reader.pages)
        
        # Add all pages
        for page in reader.pages:
            writer.add_page(page)
        
        # Add a top-level bookmark for this file
        writer.add_outline_item(
            title=filename,
            page_number=page_offset,
        )
        
        page_offset += pages_in_file
    
    with open(output_path, 'wb') as f:
        writer.write(f)