A private investigator named Tom Grant threw up this PDF on his Twitter account Wednesday -- writing ... "Download link for a free copy of the Kurt Cobain Autopsy Report at: http://cobaincase.com/Cobain Autopsy.pdf  Copy/paste the entire link, including 'Autopsy.pdf' into your browser window). This is the first time this report has been made public."

From the looks of it ... Tom has his own site that's been chronicling his investigation into the death case -- dating back to when Kurt took his own life in 1994 -- and it seems TG's come to his own conclusions about it all ... but the bigger point is this document he's published.

He claims it's the real deal, although ... he doesn't explain how he obtained it. Kurt's autopsy was never released back then -- and it all had to with some Washington state laws that prevent the disclosure of that type of info. They do release these docs to family though.

Anyway, you can take a look at the purported report yourself if you want ... because, we'll admit, if it is real -- it most certainly is an interesting read, for Nirvana fans or otherwise.

We won't get into the nitty-gritty of it all, because it's quite graphic ... but what we will say is that the alleged report purports to confirm what most everyone has known about his death for years -- namely, that he tragically died by suicide, and that he used to a shotgun on himself.

There's also info in here about what medical professionals say was found to be in Kurt's system at the time -- and it looks like there was a mix of substances. We've attempted to verify the authenticity of this report ... but King County officials wouldn't comment, and again cited the Washington law.

Kurt left behind his wife, Courtney Love, and his daughter Frances Bean. He was only 27.