Can you send a document for Shared Review once you've applied an electronic signature?

Hi! I'm having troubles sending a document for shared review, I need 3 people to digitally sign it and then send it on a shared review. I keep getting this error message however:

'The document you have chosen cannot be sent for shared review or collaboration because its current security permissions do not allow modifications. Please choose another document or change this document to allow modifications.'

I'm not sure how I can 'allow the modifications' - where's this option?

I've tried 'File/Save As/Reader Extended PDF/Enable Commenting & Measuring' but this didn't help.

We can't use stamps for signing as we want to be able to verify signatures.

Any suggestions on what the issue may be?

Thanks in advance!

Mel