If you need to quickly merge-combine PDF's without Adobe Acrobat or Apple Preview, Adobe has a free simple online tool specifically designed to complete this task.
This tool doesn't require login information for the first time you use it. However, after you've used it once, you'll need to sign in with a Google, Facebook, or Apple account.
Go to Adobe's free online PDF merge tool.
Drag and drop the PDF files you'd like to merge or click "Select files" to locate the files on your computer.
After uploading, click the "Merge" button.
Once finished, click "Download".
If you do create an account you'll get additional options, including the ability to share your file by generating a link or emailing it to others. You'll also have options to edit the file, including the ability to reorganize the pages, convert it into an alternative file format, and more.