If you have several documents that you need to scan and convert to a PDF file, here's an easy trick that uses your built-in printer software.
I'll be scanning with an HP Deskjet 3050, which is a pretty common printer/scanner/copier. I'll also be using a Mac.
1. To scan multiple pages and save them to a single PDF file, open your scanner and check the "Combine into single document" option.
2. Choose "PDF" for the format and enter a Name for the document.
3. Put your document in the scanner, close the lid and click "Scan".
Do the same thing with the next page, and you'll notice the software will say "Page #2". It will keep adding pages to the current PDF file until you rename the file.
It's that easy, and no fancy software is needed.