I frequently get asked if there is a way to see who has viewed a particular document in a SharePoint Online library. I normally tell them that they can see how many people have viewed a document, but not the specific individuals. Now, you can activate the SharePoint Viewers site feature in SharePoint Online to get that very information. Here’s how it works…
In your site under Site Settings, go to Site Actions > Manage site features:
Scroll down to the SharePoint Viewers feature and click Activate if it is not already Active:
Before the feature is activated, you can see the number of views If you hover over the Ellipsis icon:
If you click the number of views link, you’ll see the following… how many viewers by week, but no specific list of viewers:
Once the feature is activated, you’ll now see the people icons of the document viewers when you hover over the Ellipsis icon:
When you hover over the views link, it’ll show you who the viewers are:
If you click on the viewers link, you’ll get the viewers by week information, as well as the actual people who viewed the document and when they did so:
One Minute Office Magic 
If you need to go a little further and get a positive acknowledgement that documents have been read with audit reports. Take at look at Read and Understood for SharePoint on Microsoft AppSource https://appsource.microsoft.com/en-gb/product/office/WA200002742
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