If there are a number of PDF files you want to stitch together, you can do so easily with Mac OS. You will not need to install additional softwares. Here is how:
1) Open the files: Find the PDF files and double click to open them, and they will be opened in “preview”;
2) Launch “sidebar”: Click on “Sidebar” on the top menu for each of these files that have been opened. The sidebar will bring out the preview window for the PDF files. You will notice that on the sidebars, there will be small icon of the page or pages contained in the document.
3) Move pages: Drag the pages from the “sidebar” windows to one particular file (the first PDF file, for instance);4) Reorder pages (if needed): Drag and move the pages around, if needed, to make sure that the pages are in the correct order;
5) Change page orientation (if needed): Change the orientation of the pages by rotating them. To do that, click on “tools” and then “Rotate to the right” (or left, whichever takes less clicks). You can do this either to a particular page or the entire document;
6) Save the new file: Under “file”, select “save as” to save the new, merged file under a new name.