I've asked the same thing many times, and just stumbled on a way to do it easily.
Scan your multiple pages to an image file, like .jpg, I know it's tedious, name them (optional) by page number, put them into an empty folder.
When you're done with all the scanning, open the folder, select all of them, open with: Preview.
Now, you should see one page, with a side bar containing all of the pages. (If they're not in order, you can drag and drop within the sidebar to reorder.) Select all of those (Or Command A)
Command P (Or Print)
In this window, you should see page one, and below that, make sure it says "Page 1 of XX"...this way you know you got 'em all, and it's your final opportunity to page through them all by clicking on the little arrows to make sure they're in the right order.
Click the PDF button
Save as PDF...
See if that helps