Αll of your records found inside Contacts, Invoices, Bills, Estimates, Time tracking and Payments sent/received have a History option available. This is where you can find all the information about the changes a certain record has undergone through time. The system provides you with information on when a change occurred, by whom and what kind of change that was.
Each record has a History button found inside the (...) option.
Once you select History a tab opens listing all the alterations that were made, which user(s) performed them, when and more details on how the record was changed.
Every line that includes a "Details" option can provide further information for each change.
Finally, if you send an invoice via e-mail and the customer clicks on the unique permalink, then the system keeps a log that the recipient viewed the document you sent them which can be found inside the document's history.
The same is true if the client has viewed the document through the client portal, either by clicking on the link or by simply viewing the document.