In the Acrobat Reader mobile apps (for iOS, Android), you can convert from/to PDF documents using the PDF Pack online subscription service.
However, the "Combine Files" feature is not available inside the Reader mobile apps.
To use the "Combine Files" feature, you need to use the web browser interface (https://cloud.acrobat.com/combinepdf) on a Windows or Mac desktop computer.
You can sign into Adobe Document Cloud via any web browsers (e.g. Safari, Chrome) on your iPad/iPhone to view or download documents. However, you are not able to choose PDF documents to combine from the web browser interface because iOS (the operating system) does not allow users to select any local documents except images from the Photos app.
Sorry for the inconvenience.