For a determined person there are ways of converting the file to an editable word file. Even a password protected PDF can be broken by tools. You can make it more difficult and make him turn away in disgust is to use multiple columns in a page, a complex page layout and use a peculiar font which is difficult to OCR and finally by printing the document and use the printed pages to scan to PDF without OCR using a scanner. The only downside to this is the increase in file size. You can also make him run away by having more such pages like a 100 page document.
You can make him turn away with disgust by placing text underneath an image in the document which leads him to believe that this is the original document, and then he opens the original in Word. The point is that the PDF is very difficult to read, particularly on a mobile device, as it gets compressed and the quality can look different from the original version. PDFs are still useful. Many businesses still provide PDFs to their customers.