The issue is actually related to using internal PDF viewer in browsers like Chrome.
Behaviour is explained in known chromium bug:
As it is a documented browser bug, issue cannot be solved in NetSuite.
Users may try the workaround below:
1. Log in as Administrator
2. Navigate to: Home > Set Preferences
3. General tab > Defaults section
4. Tick box for: DOWNLOAD PDF FILES
5. Click Save
6. Navigate to Transactions > Customers > Individual Statement.
7. In the Print Individual Statement page, select the customer.
8. Click the Print button.
9. On the new page, please click on 'Download Statements' link
10. Click Download, or Save As
Users may also use browsers other than Google Chrome.