Where can I legally read HxH manga in English?
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.