Step 1: From the Dashboard or even home page, click on any project of which documents you want to preview. Here I am selecting Project “New Items 3370”

Step 2: Once the project opens, Click “View Assets”

Step 3: Once you can view all assets of that Project, you look for “View” option under each asset as indicated by the arrow and click on it to preview the document. It would appear as shown below. You can also preview documents from Knowledge centre as well.