How to add custom field to a pdf using Microsoft Flow

I have a Microsoft Flow triggers whenever a user creates a pdf attachment in SharePoint list. The content of the pdf attachment is different all the time. The Flow uses Adobe Sign to collect signature on the pdf attachment. My challenge is, there is one field that is common to all pdf files (let's say account number). This account number is also a column on the SharePoint list. I need to collect this information back from the signer and update the SharePoint list. For example, user uploads a pdf in SharePoint list, Adobe Sign sends out pdf for signature. Ideally, I want Adobe Sign to add an extra field (account number) at the end of the pdf along with signature field. When signer gets this pdf, he/she needs to sign and provide the value for account number. After it is signed, the value of account number will then update the SharePoint list column. I know adding additional fields can easily be done within Adobe. But is this possible to do in Microsoft Flow? Thank you.