Create PDFs, How to, Print and prepress, Standards and accessibility
What do they mean by "secure" it, exactly? Prevent it from being extracted?
If so, you can apply a security policy that allows filling in form fields but doesn't allow content extraction, but unless you also apply a file-open password it's trivial to get around it. And at any rate, someone will always be able to take a screenshot of the page and crop out the clipart, if they really wanted to... There's nothing you can do about that.
Hope you are doing well and sorry for the trouble. AS you want to secure the clip art in the PDF form but still want it to be fillable.
You can limit access to a PDF by setting passwords and by restricting certain features, such as printing and editing. However, you cannot prevent saving copies of a PDF. The copies have the same restrictions as the original PDF. Two types of passwords are available:
Document open password: A Document Open password (also known as a user password) requires a user to type a password to open the PDF.
Permissions password: A permissions password (also known as a master password) requires a password to change permission settings. Using a permissions password, you can restrict printing, editing, and copying content in the PDF. Recipients don’t need a password to open the document in Reader or Acrobat. They do need a password to change the restrictions you've set.