First, uninstall the plugin (Control Panel > Uninstall a program) and navigate to the official web page. Select your operating system and your browser version and click the Download Now button. After that, save the file to your computer, close the web browser, install the utility following the self explained steps and enable the plugin from the options menu of your browser. This should fix your Adobe Flash Player problems.
After that, download and install the latest version of Adobe Reader and configure your browser to use the plugin. If you still have issues, you can use other PDF plugins. For Chrome you can download and enable the PDF Viewer for Chrome. If you're using Mozilla Firefox I suggest you try PDF Viewer for Firefox and if you're using Opera you can download PDF Viewer extension.