Hi,
In order to do so you will need to have a password first. See here for how to set PDF password:
http://www.essentialobjects.com/doc/4/advanced/encryption.aspxOnce you have the password, you can use PdfDocument.Security.Disallow to disallow certain permissions.
With the latest build, you can also use PdfField.ReadOnly property to set just a single field to read only. For example:
Code: C#
//Pre-fill "case_number" field and set it to readonly
PdfField field = doc.Fields["case_number"];
field.Value = "12345678";
field.ReadOnly = true;
Note that PdfField.ReadOnly property did not exist in early builds.
Hope this helps. Please feel free to let us know if you still have any questions.
Thanks!