Why Windows can't do this natively
Unlike macOS's Preview app, the Windows PDF reader (Microsoft Edge's built-in viewer, or the old Reader app) has no merge function at all — you can view, print, and annotate a PDF, but not combine two files into one. That leaves most people with two workarounds: buy Adobe Acrobat Pro for one feature, or abuse the "Print to PDF" driver by printing each file into a new combined document, which quietly strips out clickable links, form fields, and text searchability along the way.
A dedicated browser-based merge tool avoids both problems — no purchase, and no loss of the PDF's original structure. Prefer a full walkthrough with the Print-to-PDF workaround explained step by step? Read the complete guide to merging PDFs on Windows.