Usually, this option is available for scanners with a document feeder. It is possible to do it with flatbed scans using some software, but I forgot which scanners came with that software and what the title was. When I need to make a single PDF from multiple items, I normally scan to single files and "post process" using the
Bullzip PDF printer (virtual printer driver). That software is free for personal use and small business, and allows you to add prints to existing PDF files in several ways (prepend, append, overlay, watermark, underlay). Undoubtedly, there are other software titles with similar features.