I guess the same way as on other platforms: using Acrobat, or the PDF Pack service.
[topic moved to Reader/Acrobat DC for Mobile forum]
Adobe PDF Pack (paid subscription service) provides the capability to combine files.
Alternatively, you can use Adobe Acrobat Pro (desktop app) to combine files.
However, the Reader app on iPad/iPhone does not have the in-app support for combining/merging PDFs.
Sorry for the inconvenience.