You can use the Preview, but it mainly provides simple editing. If you need further editing, you can consider downloading a PDF editor of Pdf Merging Online (not free, but has a 7-day free trial), or search for EasePDF and ilovepdf (both are free to use) on Google. Many online PDF editors are free to use, but some may have restrictions on use, so if you often need to edit PDF, you can consider joining a member.
Click here to see a list of all the online PDF merging software in one spot.