You might already have LibreOffice installed. Its Draw component can merge PDFs, though the process is less obvious.
cPDF is a single, tiny executable (no installer required). It runs from command line/terminal – perfect for users who merge PDFs regularly. freeware to merge pdf files
PDFsam is the gold standard. The Basic version is completely free and does exactly three things: merge, split, rotate, and mix pages. No ads. No watermarks. No registration. You might already have LibreOffice installed
PDFsam Basic (Windows/Mac/Linux). Offline, no watermark, open source. Drag, drop, run. Avoid online tools – they’re privacy risks. PDFtk Free (Windows) and cPDF (command line) are excellent alternatives. All completely free, forever. open source. Drag