I downloaded an EPUB file, how can I get it from Ubuntu to my IPad?

I would use Calibre. It is an awesome and easy to use Library management system. You can install it as follows:

sudo apt-get install calibre

I haven't used it with iPod/iPad but I use it with my Android phone and Kindle all the time and it is great.

EDIT:

I might also add that the iPad driver is still experimental, so you may encounter a few bugs. It is better than nothing though

Thanks to Kenneth for suggesting adding the following link which helps greatly!

http://manual.calibre-ebook.com/faq.html#how-do-i-use-app-with-my-ipad-iphone-ipod-touch

You could use a cloud drive. Upload the files to the cloud drive from your pc and download them again on your ipad (for example dropbox). You could also send a mail to yourself with the eBook as attachment.

If you would like to know more about sending files to your ipad i would suggest reading the following article, it covers multiple options on sending files from your pc to you ipad.

http://www.macworld.com/article/1159805/iPad_file_transfers.html

If you email it to yourself as attachments that you can download, so that it's on an email address on your iPad, you can open the ePubs and it'll add them to the books feature. Assuming, of course, you're on the latest iOS.

I have yet to find a direct-from-linux-to-iOS solution though... the above does work though.

I have found a workaround that works really well for loading ebooks onto your iPad, and you dont need internet access at all. Download the app USB Disk for your iPad (it is free). Plug your iPad into PC. Navigate to your iPad in the file explorer. The USB Disk app should show up. It will have a folder called "Documents" inside of it. Drag all of your ebooks into this folder. They will then show up inside the USB Disk app on your iPad. You can then choose the option "Open with app" --> iBooks, to add them to your library.

This might be the most hacker(hard) way to do it.

Close your ibooks app, do not open it till the end! Extract your .epub file in to a folder with the same name. If the file is book1.epub, the extracted contents should be in the folder book1.epub.

Open the iDevice in your Ubuntu file explorer. Navigate to Books/Purchases copy the extracted folder here.

Open the purchases.plist file in your text editor. It's an xml.

It should look like this

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Books</key>
    <array>
       <dict>
            <key>Artist</key>
            <string>Chuck</string>
            <key>Inserted-By-iBooks</key>
            <true/>
            <key>Name</key>
            <string>Fight Club</string>
            <key>Package Hash</key>
            <string>6744918BA88D5BF234A2B663F3A311E2</string>
            <key>Page Progression Direction</key>
            <string>default</string>
            <key>Path</key>
            <string>Fight Club_ a novel - Chuck Palahniuk.epub</string>
            <key>s</key>
            <string>0</string>
        </dict>
        <dict>
            <key>Artist</key>
            <string>George Orwell</string>
            <key>Inserted-By-iBooks</key>
            <true/>
            <key>Name</key>
            <string>The Complete Novels Of George Orwell</string>
            <key>Package Hash</key>
            <string>6744918BA88D5BF234A2B663F3A311E1</string>
            <key>Page Progression Direction</key>
            <string>default</string>
            <key>Path</key>
            <string>Complete Novels Of George Orwell, The - George Orwell.epub</string>
            <key>s</key>
            <string>0</string>
        </dict>
    </array>
</dict>
</plist>

Copy a section between with the <dict>...</dict> tags and fill in your meta data of the book.

For the Hash just change the last two characters, so that it doesn't duplicate any other entry. Save the file and open the iBooks app.