Unable to copy text from a PDF file created by WPO2020 (and earlier)

I have created a lengthy reference manual for my office. It includes a ToC, index and hyperlinks throughout the manual. All good.

My staff do not have access to the original WP document, they work only with the PDF version of the manual, for obvious reasons..

When I publish to PDF, the document is created correctly and all the hyperlinks work just fine.

On occasions, my staff needs to simply copy some text from the PDF manual to paste in to another document/email or similar.

However when you open the PDF manual published by WP, you are unable to select text with any system PDF viewer.

There must be a setting in the PDF publisher which is disabling copying from the PDF.

I can use various PDF writers (Nitro, Microsoft) to create the PDF to get around this, however these do not support the hyperlink function, which is important.

Does anyone know if there is a solution to this issue?