I'm going on a trip and want to read some manga on the airplane.
So I was wondering if there is a way to legally download manga to a PC?
As mentioned by one of the commenters, you're going to just have to go and buy digital versions if you want to read it offline on your PC legally.
I took a look at Viz Media's Hunter x Hunter web page and found the places where they sell the manga digitally. Of the four sources listed, I found that Amazon and Nook sell Hunter x Hunter at the cheapest price of $4.49 per tankobon volume. Amazon has an actual desktop application for both Windows and Macs, while Nook only has a Windows store application, so I think Amazon might be the better way to go if you do not have Windows 8, 8.1, or 10.
You can buy it on iBooks Store.