(I usually save as notepad file, but not sure if Mac has an equivalent)
You don't need to associate it with any particular app to save it to your Mac. As Dale says, just click the download button and it will save the file to your Downloads folder. (To find the file on your Mac, open a Finder window and there should be a link to the Downloads folder in the Favourites list on the left side of the window, otherwise from Finder open the Go menu and select Downloads, or use the keyboard shortcut option+command+L)
I then wanna upload to my ipad for walking next week.
Or maybe I can do it 'straight through Viewranger ?
As Kim says, you need to do it via an app, either on desktop or device.
I don't know Viewranger but I've just been looking at it - very interesting. The app does allow you to import files directly
1 but you need to have the file saved in your iCloud Drive or in Dropbox (you'll need to link your Dropbox account to the app to allow this).
The default option on OS X these days is for everything on your Mac Desktop to sync to iCloud Drive
2, so if you drag the downloaded GPX file from the Downloads folder to the Desktop, it should then appear in the Desktop folder when you select Import File on the Viewranger app. I've just tested this and it works a treat. In order to use the file for navigation, you then have to convert the Track to a Route in the app.
You can also upload the GPX file to your account from your Mac by logging in at viewranger.com and selecting Routes and Tracks > Create a route from GPX
1. Unlike, say, Strava or RidewithGPS, which require you to upload files to your account on your desktop so that you can access them through the mobile app. Thumbs up to Viewranger for this excellent feature.
2. If your Mac isn't set to sync to iCloud Drive, open System Preferences > iCloud, then tick the checkbox next to iCloud Drive. Also click the Options... button next to iCloud Drive and tick the checkbox next to Desktop & Documents Folders.