You can Certify the PDF form before placing it online. You will be able to set the "Permitted Actions After Certifying". The users won't be able to change the PDF form, only the owner of the digital ID who certified the document.
If you don't have a digital ID/certificate you can create one with Acrobat, by going to menu Edit (Windows) or Acrobat (Mac) -> Preferences, choosing "Signatures" category and clicking on "More" under "Identities & Trusted Certificates" (Acrobat XI).
To create a digital ID:
To certify a PDF document, once you have a digital ID, go to menu View -> Sign, open "Work with Certificates" panel and you can click on Certify (Not Visible)