Hello,
Adobe Acrobat is used to design and edit forms, while Adobe Reader is only for reading.
As you have stated that you have created form using LC designer and clients are using Reader to open it.
If you really want the clients to edit/fill the form you need to reader extend the form( to give usage rights)
Now this can be done in two ways using Adobe Acrobat(paid desktop tool) or Adobe LiveCycle Reader Extensions service(Server product).
Once you reader extend any form, actually you enable some usage rights on that particular form and once you open that form in Reader, you will be able to perform the action that you have enabled.
Once the form is reader extended, users can fill, edit sign(the rights you have given).
-Vijay