Create a basic Document Management system using QT

I have not much knowledge of Object Oriented programming, I mostly do C code. I struggle with creating UI interface with multiple form talking to each other. I also have problem with implementing a database backend to a multi form Interface. I manage to do it, but it far from the right way to do it I'm sure and cause issue when I try to expend the application. So I'm looking for help in creating a well made structure that I can either continue to work on, or maybe at that point let someone else continue it if I have a good experience. I've create a small sample app to show a few form to give an idea of what I'm looking for right now visually. I basically want to store and display PDF document. Now I don't need help with that part since I've already made a class that do most of what I want. You can simply load a ascii text file so you don't waste much time on it. By default they are unclassed, and once open, we need to be able to associate them to business. If business doesn't exist, be able to create it. There is different type of document that have different fields associated to them (invoice:date,account number, subtotal,taxe,total, contract: title, date signed,date effective, date expire)... They can be static in a local xml file or into the database so they can be expanded on. The idea is you select the field subtotal and then you select the text inside the document that have the subtotal. In the DB, you store the position of the text (line number, character position on that line, and the end line number, and character position of that line (I basically do the same thing with the PDF document, get the X,Y position of the top left, and bottom right so I have the box coordinate of the content) I've attached also a sample SQL db structure. I also wish the code doesn't interact directly with the DB, since I'll want to move to a Restful API in the future, so I rather that there is a seperate class that is use to get the data, and can be easily change to return value from different type of data source. So I'm looking to make the attached sample GUI into a functional and expandable application. Where form talk to each other when needed. Where they can access data, and store. I do not need an import document feature. You can pre populate the database. The actual local filename are based on the ID of the document. So document id 1 would be [login to view URL] I'm open to changes in the UI that I made, I just did this as a quick example of what I want. The mainwindow contain the search for document and display document right in there... But in the future that not going to be like this. So if you create an empty mainwindow with menu or button to access the document search feature, that would be fine with me too. Please if you need something more details and simplified as step by step execute like I think it would be needed, let me know !