Open a PDF merge tool (online or desktop), such as Adobe Acrobat, PDFsam, Smallpdf, iLovePDF, or similar
Choose the option to “Combine” or “Merge” PDFs
Add the PDF files you want to unite
Reorder files if needed
Select “Merge” or “Combine”
Download or save the merged PDF
(Alternative) Use macOS Preview
Open the first PDF in Preview
View thumbnails (if needed)
Drag additional PDFs into the thumbnail sidebar
Reorder thumbnails as needed
Export as a single PDF (File → Export as PDF)
(Alternative) Use Windows with Microsoft Print to PDF
Open each PDF and print to PDF into one document (may not preserve all layouts perfectly)
(Alternative) Use a command-line tool (PDFtk or similar)
Run the merge command to output a single combined PDF file