I have recently bought a laptop with Windows 7 installed. Being involved with a local charity (VAT registered) I have tried to download past VAT returns which should appear as PDF files.
The response I get from the HMRC website is "Unable to download return - unable to open this internet website. The requested site is either unavailable or cannot be found". I cannot get any response from HMRC.
I also had a problem downloading .pdf files from the HMRC site - not quite the same as yours but similar. I don't think it's a problem with Adobe Reader, but rather with the HMRC setup. If your Adobe Reader opens other .pdf files OK then try the solution they offered me (found their helpline very 'helpful' by the way).
In Internet Explorer go to Tools | Internet Options | Advanced | Security | tick the box beside Do not save encrypted pages to disk. Click Apply and OK. Then try to download again. Hope it works for you as well.