Usually, whenever you scan a document onto your system, OS X will create separate files for each subsequent scan which you can save to the location of your choice. This can be an issue if you want to merge many scanned pages into one document, for either sending to someone or for storage. Luckily, OS X does support combining scanned pages into one document. Read on to find out how to do this:

There are two different ways to combine two scanned documents into one document. You can either directly combine your files while scanning them so that they are all directly scanned into one document, or you can combine your already scanned documents. Both methods will use Preview in OS X.

Full article here:
How to Easily Combine Two Scanned Documents Into One in OS X (Make Tech Easier)