You can certify your PDF (provided that it does not have any signatures yet). However, the permissions granulation in Acrobat UI does not allow you to distinguish between signature fields and other field types. So, if you allow others to sign this PDF you also allow them to fill other form fields, which is not what you want. You need LifeCycle solution that has more granular permissions handling but this is an expensive piece of software. If you distribute many PDFs like this it may make sense for you. If your volume does not justify this investment you can do the following:
If you do not have other signatures you can flatten your PDF form (Google "flatten PDF form in acrobat x" to learn how to do it), add one or several new unsigned signature fields, certify with "form fill-in and signing" permission (this PDF does not have any form fields other than signature fields anymore), so that the content cannot be changed and send it to the signers.