Lock Buttons in Adobe DC

I have a PDF with buttons that allow the user to click them and insert an image as an icon within the button. I now want to lock these buttons so the images can no longer be changed. What's the best way to do this? I have looked into flattening the file, but I can't seem to do it. I'm running Adobe DC, and under "Save as other" I do not have the option to "Optimize PDF", which is how I understand you would typically flatten a file. Any assistance is greatly appreciated!