What is the recommended pack? Do I have to install it?
The recommended pack is about 9.5 GB: Japanese vocabulary, pitch-accent and frequency dictionaries, plus local Japanese / English pronunciation audio libraries. If you are learning Japanese, install it and save yourself picking dictionaries one by one.
How to install
Easiest: in the "Recommended pack" step of the onboarding wizard, tap Download and import. It downloads from several sources at once in the background and imports automatically when done; you can cancel at any time and resume later. The download keeps going in the background, so you can move on or close the wizard — progress, cancel and import live under Settings → System and about.
You can also download the pack to disk from the download page first, then come back to the "Recommended pack" step and tap Choose a pack file. In the confirmation dialog pick "Merge into current library" — that only adds the dictionaries and pronunciation libraries and leaves your data alone; "Overwrite entire library" wipes what you already have.
Slow download
The recommended pack section of the download page has chunked direct links for IDM / aria2 and similar download managers (download the parts and join them in order), and a single-file copy on Google Drive.