PDF Share Forms Information Worker supports two different way to store attachments.
In general, Adobe allows adding attachments in static forms opened in a browser. You can find how it can be done in the guide (Storing attachments inside PDF form). This approach has limitations – files that are attached to the form this way, can be reached only through the particular form and are not accessible through SharePoint.
In case, if you need having attachments as separate documents in a SharePoint library, we could suggest the following scenario: configure template in a way, that forms and attachments will be submitted into the folder (Storing attachments in the same folder as PDF form). These attachments will be available in SharePoint and can be used in workflows and other SharePoint activities.