I think it is a built-in feature of Google sites that they send Firefox a PDF instead of providing a specially print-formatted page that you could print to the printer of your choice. I don't know why, but I assume this was how they could give users the best results. (If you are printing from an open Google Sheets document, the first screen you come to shows Sheets options, similar to MS Excel. That's not the standard Firefox print experience -- Sheets intercepts Command+P.) Unfortunately, when a PDF is served as a download, it is always saved in some folder instead of in the web content cache. On Windows, the first two options would use the "Temp" folder, which the system cleans up in the background. But wherever they are saved, you're right that they get listed as downloads in Firefox's download history.