What is the fastest way to spread a free report ?

I don't know about "fastest", but in the "whitehat" vein - submit your ebook to the various "free ebook" sites on the net. These are sites like scribd, manybooks, bookcrossing, word public library, etc. Google it and you will find lots of lists on the internet - here is one:
Best Places to Get Free Books - The Ultimate Guide

In the regular "whitehat" vein (and adding on to Jennifer's post above) you can find forums related to your ebook and just post in their "resources" or "free ebooks" or similar sections. I think WF (if your ebook is about marketing) still has a section like that too, doesn't it? Certainly DP does. Yes, Jennifer's idea of creating controversy will accelerate the distribution of your book, but a GOOD title, with a GOOD post selling the free ebook works too.

In the "blackhat" vein - create a torrent of your ebook. Assuming it has a compelling title, and you can write a compelling description, you will get plenty of people DLing it, thinking they "got one over on you." Doesn't work as well as it used to (been done a lot by others) but still works. You just keep seeding torrents at the larger torrent portals (Mininova, Demonoid, etc. etc.) and you will keep getting downloads. Further, if your free ebook is good, those folks will continue to seed your ebook for quite a while. The various P2P clients are also good to use for that (the ones with private networks, like emule).

In the "greyhat" vein - create a small site dedicated to your ebook. Make it look like you are selling it. Create a download page that looks like something someone would reach when they buy a product. Then, just skip the actual paypal part - link directly from your "sales" page to the download page. Again, some folks will think that they "got one over on you" and if you do it right, your ebook will be posted in all kinds of blackhat forums for download. You can always help that along by signing up at one and posting the download page yourself.

For the "traditional" approach, write a press release about your free ebook, and submit it to as many press release sites as you can. That gets an initial flood of downloads, assuming your ebook is well-titled and covers a popular topic.

Good luck!

Best Regards, Georgetta