Sorry for the delay in response.
As you mentioned above, when you send the form for signature, the recipient need the password to open the file.
Could you please let us know the how exactly you have created the form.
Let us know if this issue occurs with a particular form.
Did you try to send the form again?
Is it possible to share that file with us?
You can share the file using the steps mentioned here How to share a file using Adobe Document Cloud
Also, share the dot version of the application and OS you are using.
Take help of the steps mentioned here Identify the product and its version for Acrobat and Reader DC to check the version number of the application.
Adobe Acrobat and Reader official update containing the fix for this issue is now available. This update will be automatically pushed to all existing installations of Acrobat and Reader. If you want, you may also manually trigger the update by opening the application and going to Help > Check for Updates.
More information about this release is here: Release notes | Acrobat DC, Acrobat Reader DC, Acrobat DC Classic 2015, Acrobat Reader DC Classic 2015, Acrobat 2017 and…
Please check and let us know how it goes.
Let us know if you experience any issue.