Can't edit layers on a read only pdf

I've created a digital pdf product which has many layers. Customers can edit and remove layers they don't need. I've recieved some feedback from some customers saying they can't access the layers panel because the pdf is set to 'read only'. 

I have it set to read only so that it's harder to copy my product. My understanding is that you can still access and edit the layers on a read only pdf - is that correct? 

I'm just wondering if the issue these cutstomers are having is due to something else (outdated software, file getting corrupted etc)? Some customers are able to edit the layers which is why i think it must be something else?