With Documents, you can easily merge two or more PDF files into one without even opening them - if needed, you can re-arrange the pages manually in your new file to your liking. As for the other file formats, you can convert and merge them into PDF.
If one of your files or all of them are forms, you may receive an error while trying to merge them. There's a way you could bypass that: if you want to merge PDF forms, please save them as flattened copies first:
If one of your files or all of them are forms, you may receive an error while trying to merge them. There's a way you could bypass that. If you want to merge PDF forms, please save them as flattened copies first: