There are three basic ways you might want to use the Merge feature: either merge pages or merge entire PDF files.
Option 1:
1. Click Tools on the toolbar > Merge;
2. Add Files you want to combine;
3. Set the page range and page size as you desire.
Option 2 (Mac Only):
1. On the main menu, click File -> Merge PDF Files;
2. Select the PDF file(s) you want to add to the existing one and press Open;
Option 3 (Windows Only):
1. Click File at the upper left corner -> Merge PDF Files;
2. Select the PDF file(s) you want to add.