Unfortunately this is not possible at the moment. The reason is security.
Due to data protection, clients need to fill in questionnaires in one go. If it was possible for clients to save their progress mid-way whilst completing a questionnaire, then anyone with the link to the page would be able to steal their data.
The way NutriAdmin is structured, data provided via questionnaires is always secure. When a client clicks on "submit" a questionnaire, data is sent via a secure TSL protocol. This means that data transferred between your clients' web browser and NutriAdmin's database is encrypted. TSL (newer version of SSL) is the standard of web security used by most web applications managing data, and you can easily tell by checking on a website's URL (it will say https, as opposed to just http).
In the event of someone other than your client gaining access to the link to a questionnaire via illegitimate means, they would see an empty form. On the other hand, if your client completed the questionnaire already, the link to the questionnaire would expire, and anyone accessing the corresponding website would just see a message saying "this questionnaire has already been submitted". This way, even if someone stole a link to your client's questionnaire, they would never see any data at all.
In the above scenario however, if a questionnaire was partially completed, your client's data privacy would be at risk, and NutriAdmin would be infringing it's security policies. This is why we don't allow clients to store their progress mid-way through a questionnaire. It would be practical, but not secure.
Finally, a couple of possible workarounds to alleviate this limitation would be to: a) Create a couple of different questionnaires, some of them shorter if you want to send separate chunks to clients. b) Asking your client to download the website in their screen whilst working on the questionnaire, and then to re-enter the data manually.
The above workarounds are perhaps not too practical or ideal, but security must come first ahead of practicality when dealing with confidential data.