I am having this issue also.
do you have a link to this issue in your website? also, http or https?can you view or download the attached pdf from earlier in this thread?
is your browser updated? chrome will not display PDF or flash or other add-ons if you are using an outdated version of chrome -- restart your browser.
what browser add-ons are you using? are they blocking Google servers or third-party servers... such as Google's?
does this happen when you try viewing on a different browser?
these are the first steps in debugging your issue.
EDIT: Have you tried the .httaccess steps as previously discussed in this thread?