PDF+PDF is a very useful widget which allows you to merge multiple PDFs in one in just a few simple steps. You just need to copy the documents you wish to merge in Finder, open the dashboard, go to the application, and paste them there. Then, you click on the equality ("=") sign, and the resulting document will be added to your desktop.
The program allows you to merge the complete documents or you can select a range of pages to merge. Sadly, you can add 4 documents at most. But if you need to merge more, you can merge them later with the resulting document. You can also rearrange their order by simply dragging them within the widget. You can also easily delete files, if you make a mistake. What is more, the name of the resulting document can be easily changed by just typing it in the corresponding field. When the process is complete, the files on the widget will get a tick next to them.
All in all, if you are looking for a simple and nice application to merge multiple PDF files in one, PDF+PDF may be a good choice.