Depending on the type of scanner you have, you might only be able to scan one page of a document at a time. If your scanner saves files as PDF (Portbale Document Format) files, the potential exists to merge the individual files into one document.
Open Adobe Acrobat Pro.
Open one of the documents that you want to merge together with other documents.
Click once on the "Document" drop down menu and select "Insert Pages."
Select one the PDF files you want to you want to add to the original, and click once on the "Select" button. (It helps to save all of the PDF files that you want to join together in one folder.)
Use the "Location" drop down menu and "Page" section on the "Insert Pages" pop up window to determine exactly where in the document you want the additional PDF added. Click once on the "OK" button.
Repeat Steps 3 through 5, as necessary, to continue adding pages to the document.
Save your new document under a new name to avoid writing over the original, smaller document.