Hi Mvalade,
As you have mentioned above, you use the Fill & Sign option in the Adobe Sign on your desktop.
The Fill & Sign option that you are using on the mobile device is different from the one that you use in Adobe Sign. That is the reason it does not lock the document.
The Fill & Sign tool for Adobe Sign is not available in the mobile application.
Could you please confirm if you are using the Fill & Sign application or the Fill & Sign tool in Adobe Acrobat Reader application on your mobile device to fill the PDF form?
I would suggest to use the Adobe Acrobat Reader application to add signature on the PDF form. It will not lock the PDF, however, after adding a signature and saving the form, it will not be possible to make any changes in the form. That means you will be able to add new information, however, cannot edit any information already in the PDF form.
There is a workaround you can use. Open the Adobe Sign on a browser on the mobile device and then you can use the Fill & Sign tool provided there.
Hope the information helps.
Let us know if you have any questions.
Regards,
Meenakshi