As per the issue description mentioned above, you are facing issue when trying to send pdf for signature, is that correct?
If you are using Acrobat or Reader try logging-out and login-in back, check if that resolves the issue. If you still experiencing the problem you may use this service via web browser: Sign In — e-signature and e-sign Software Solution — Adobe Sign
Also, make sure your web browser is updated to the latest patch. You may refer to the following help docs for more info:
Let us know if you need any help.