No, you cannot create any form of categories under tabs. But you can attach files to the transactions. And you can create separate folders under the Folders tab to store files. You can also change the title of sales quotes to whatever you wish, one by one. So you can label document as a Sales Contract or as a Pro Forma Invoice. But they will all be in the same list. And none have any financial effect.
just create a custom field for the document type with a drop down list shown as a column and not printed on documents. the drop down list can contain headings such as MOU, Proforma Invoice as defaults. now you can select these headings in any document and they will be shown as a column in the tab which will be easier for you to search by heading.