When you scan a document into your system, OS X will create a single file of the scan and allow you to save it to the location of your choice. However, if you would like to combine scanned pages into one document, the system supports that option as well.
The first approach is to have the system scan a set of documents directly into a single file, which can be done by using the basic scanning features in Apple's Preview program. With your scanner attached and turned on, open the Preview program and select your scanner from the File menu. The scan import window will appear and will give you options for setting the resolution and color depth, page size, orientation, and other properties.
Of these properties, one is a small check box called "combine into single document" that will be available when you select some formats like TIFF and PDF. By checking this box, you can scan an individual file, and the imported image will be placed in an untitled file. Subsequent scans will then be placed in this same file as long as the check box is activated. Unfortunately you cannot use this check box to append pages to an existing multipage document; however, you can use Preview's page management tools to assemble individual scans into a single document.
If you have multiple scanned documents that you would like to combine, you can do so by first ensuring that they are PDF documents. If they are JPEG or PNG files, convert them to PDF by opening them in Preview and then using the Export feature in the File menu to save them in the PDF format. When done, you can then open one to use as a master copy in which you will place your other scanned images.
With the master PDF file open in Preview, select Thumbnails from the View menu; you should see a list of page previews next to the main content area. Now you can drag a page's thumbnail from one document to the location of your choosing in the master PDF document and then save the master to have a compiled PDF of all of your scans.