There are several possible causes for this issue, but let's just take them one by one. So, you are saying that your PDF reader functions normally, excepting the web mode. The most probable reason is the fact that the option of using the PDF reader in association with online documents was disabled. From your question, I understand that you are using Internet Explorer as your default browser. If so, make sure that the AdobePDF.dll is enabled(Tools> Internet Options> Manage Add-ons> Show menu to "Add-ons that have been used by Internet Explorer"> Find and select Adobe PDF Reader> Enable> OK). Second thing you can try is to repair Adobe Reader. Do that by opening Adobe Reader, and hitting Adobe Reader Installation from the Help menu. Try to open a copy of your PDF file in the web browser(open your browser> File> Open> File of Type> All files> Browse> Load your pdf file). Deleting the temporary files and the cache of your browser is also a good idea. Another frequent situation is the one caused by Internet Explorer 64bits version. try to download and install the 32bits version.