To add to this, click on the Great Lakes section, and download the Catalog. Then you can look at exactly which ones you want. I'm in Lake Michigan, so for example, I usually want 14907. I then click on the 'download RNC' in the upper right once the chart appears.
I typically use
this page to access them, because I don't need to preview the charts online, and this allows me to download the ENC or RNC as I am in the mood for.
I've been experimenting with the Vector (ENC) charts as well, which are available from the other section of that page I note above, but OpenCPN handles them nicely. They are purported to be more accurate and easier to read, but I've never used one live on the boat. Hope to change that this summer!