I have created a fillable PDF form with a digital signature block. I am using Acrobat X Pro. I have saved the form as a Reader Extended PDF and Enabled Additional Features. In testing the form, some users are able to successfully digitally sign, but others are not. I have gone back to ask which version of Adobe Reader they're using (those who can't sign). Is anyone aware that enabling those features will only support certain (recent) versions of Reader? I believe opening the form in Apple's Preview program would be a problem as well -- they'd need to actually force it to open in Reader. Am I missing something else? Thanks in advance.