Exporting to PNG (or JPG) is good way to protect your document from copying (instead of PDF with poor password protection). Very easy to implement. There should be checkbox with "All pages" in export menu. Now it exports only current page.
Why does it protects? If you will use complicated fonts even OCR would not be able to recognize tyour document.
Marked as enhancement request
Hi, in case of Impress (as mentioned on the vote for enhancement page on the document foundation wiki), it's maybe better to implement the possibility of creating a picture presentation as in PowerPoint. See :https://bugs.documentfoundation.org/show_bug.cgi?id=93589