You cannot download as much data as you like, there are some restrictions, again, from the same source as above:
Google has incorporated quota limits for Trends searches. This limits the number of search attempts available per user/IP/device. Details of quota limits have not yet been provided, but it may depend on geographical location or browser privacy settings. It has been reported in some cases that this quota is reached very quickly if one is not logged into a Google account before trying to access the Trends service.
Click through to see how you can access this data. In this case, the example focuses on specific categories but there’s a lot more within Google Trends.