If you have received a completed PDF Form that you want to convert so it is no longer a naturally editable form then you can follow this guide to set the PDF Form Fields to be Read-Only using the Form Tools on Adobe Acrobat Pro XI.
Open the PDF in Adobe Acrobat Pro XI
Open the "Tools" menu and then click the drop down menu for "Forms" and then click "Edit"
You should now see the Form Fields editing options as shown below.
Next, you need to right-click one of the Form Fields and then click the "Select All" option (The shortcut key "Crtl + A" is another good way to Select All)
After you have all of the Form Fields selected/highlighted, right-click one of the Form Fields again and then click "Properties" from the same menu shown above.
In the "Text Field Properties" dialog box that pops-up, click the "Read Only" check-box and then click "Close."
Your PDF file now has the Form Fields set to Read-Only and you can re-save it if necessary.