PDFsam Basic is useful when the task is about page order, splitting or combining files. Keep source PDFs unchanged and check the resulting document, particularly when mixing alternate pages from separate front and back scans.
Work from copies of the source PDFs and check page order before running a merge or mix. Use a small sample when combining front and back scans. Open the resulting file to verify orientation, bookmarks and completeness before deleting intermediate files or sharing the final document.