Sorry for the delayed response.
As per the issue description mentioned above, you want to create a letter and sign it online, is that correct?
For creating a pdf or letter, you would require Adobe Acrobat where you can create a blank pdf page. As we have checked the account details using the email address you are logged into the forums, you have a subscription for Adobe PDF Pack. Refer to this help document for more info on | Adobe PDF Pack
You can however, open any pdf file or pdf form in Adobe Reader free desktop application and sign it using fill & sign or using a digital ID.
To download Reader, visit this link: Adobe - Adobe Acrobat Reader DC Distribution
Refer to this link on how to use fill & sign: Using Acrobat Reader DC Fill & Sign
Let us know if you have any query.