Does a test PDF document which says "Congratulations" appear below?
No? You'll need to enable your browser's PDF viewer. Google your browser's name and PDF Viewer (for example, chrome pdf viewer) for instructions for your browser. When you update your settings, then reload this page and see if the test document now appears.
Does your browser have third party cookies enabled?
If you can see the test PDF document above but still cannot view PDFs of ebooks or articles, you may need to enable 3rd party cookies. Click this link to find out if they are enabled:
Third-party cookies test
No? See how to enable cookies and change the setting so that your browser does NOT block third party cookies. Then try accessing the ebook or article again and see if it now appears.