Q:

How to watch movies on iPad? 

Here is the setup:
I have an iPad air 2 and a server in my homenetwork that runs torrent client Transmission and a Plex-server. 
I use the server to stream media to the iPad while i'm home, direct media to my chromecast etc. 
I need to solve two issues:

Sometimes I want to add new torrents to the transmission-client on the server.  From a computer, I currently do this using the transmission web-service. 
Sometimes I want to download movies to my iPad and watch them while I'm not home. 

A:

Plex Media Server offers "sync" functionality to devices if you have a Plex Pass subscription (starts at $3.99/month, less per month for longer subscription periods).  You initiate the sync either from the Plex web app or device by selecting movie(s) to sync, and it then transcodes your selected media into a format appropriate for iPad.  You then open the iOS app to download it directly to the device.  See Quick Guide to Plex Sync for a run-through. 
For adding torrents, the best solution I've found is to use Dropbox.  If you don't already have a Dropbox account, set one up.  Download the Dropbox client onto your server. 
In the Mac menu bar, a new icon of an open box will appear.  Tap on that, then tap on the folder icon to the left to open the Dropbox folder.  Here, create a folder called "torrents" (or whatever you like). 
In Transmission, go to Preferences > Transfers and set the Auto Add directory to the Dropbox directory you just created, which will be stored locally at /Users/yourusername/Dropbox/torrents/ or whatever you called it. 
On the iPad, install the Dropbox client.  Next time you want to add a torrent, act as if you're downloading it to the iPad.  Since you can't actually download files to the iPad, the screen will change to show the filename, and will have a "Open in Dropbox" link at the top-right.  Tap on that, then select the torrents folder you created in Dropbox.  Tap Save at the upper right. 
After a few seconds, the file will sync to Dropbox from the iPad, then from Dropbox to your server.  Since Transmission is watching the directory where it will appear on your server, it will be picked up and automatically added. 
There's a few steps involved in setting this workflow up, but once done, it's maintenance-free.