Use an online PDF combiner tool (upload files, reorder if needed, merge, download)
Use Adobe Acrobat (File → Create → Combine Files, add PDFs, reorder, Combine, save)
Use macOS Preview (open PDFs in Preview, select thumbnails, drag to a single document, export as PDF)
Use Windows “Print to PDF” (open each PDF, print to a single PDF output, combine via repeated prints)
Use PDFtk (run `pdftk file1.pdf file2.pdf cat output merged.pdf`)
Use Ghostscript (run a merge command to concatenate PDFs into one output file)
Use command-line tools like `qpdf` or `mutool` (concatenate PDFs into one merged PDF)
Use a programming library (e.g., PyPDF2/PyPDF for Python, iText for Java/.NET, PDFBox for Java) to append pages and write a single output PDF
Check page order and bookmarks after merging
Save the merged file under a new name to preserve originals