I have a lot of Google drive's file download links in my blog. So should I place
rel='nofollow' in Google drive's file download link for good SEO?
You don't need to put nofollow on any links which are perfectly trustworthy, It's not needed, there's no reason to try and sculpt your page rank if that's what you're aiming for.
From SEOMoz and Matt Cutts:
Does the Number of Outbound Links from a Page Affect PageRank?
For instance, to conserve "link juice" and/or funnel it more discretely, does it matter whether I have three outbound links versus two? In the original PageRank formula, yes, juice flowed out in a simple formula of Passable PR divided by number of outbound links. But nowadays, Matt says it is a much more cyclical, iterative analysis and, "it really doesn't make as much difference as people suspect." There's no need to hoarde all of your link juice on your page and, in fact, there may be benefit to generously linking out (not the least of which is the link-building power of good will).
On the other hand are your trying to hide the download file from everyone but authorized users? If so you should be going about it another way using a script to initiate the download when the user is authorized.
If the file doesn't matter just link to it.