I have a few processes in Workflow that export an entry out of the repository and to a file path on the server.  It's done by an SDK Script activity, which I'll include below.

Since you said it is from a form, I would recommend having the form first use the Save to Repository Service Task to create the entry in the repository, and the Workflow Service Task to call your Workflow.  Have it save it in PDF format, because the script below is assuming it is an electronic document attachment on the entry.

If you are on version 12, you should be able to pass the values from Forms to Workflow regarding the entry that was created earlier in the Forms process (such as the entry ID).  If you are still on version 11, then you will want your Workflow to search for that entry in the Repository (in which case, you may want to do something like having the Save to Repository Service Task include the Forms Instance ID # in the fields, so that it's easy to search for via that value).  Either way, you'll likely need Either a Search Repository or Find Entry activity to get the specific entry(ies) and a For Each Entry activity to loop through the result(s) from the Search/Find.

Once you have identified your entry in Workflow - you'll need a token value named File Path with the value of that being the full file path and file name (with the .pdf extension included).  The script is going to use this token - I do it this way to make it easier to manage/edit since a token is easier to work with than the script.  It's also easier to use other tokens within the File Path token.

Then you can run this script within an SDK Script activity, set to C#, and selecting the entry that you identified earlier in the workflow: 

        protected override void Execute()
        {

            // Get document
            DocumentInfo doc = (DocumentInfo)this.BoundEntryInfo;

            // Initialize Document Exporter
            DocumentExporter dExp = new DocumentExporter();

            // Export electronic document
            var filePath = GetTokenValue("File Path").ToString();
            dExp.ExportElecDoc(doc,filePath);

            // Cleanup document object
            doc.Dispose();

        }