Can I save a document to a CDT?

I think this has been a behaviour/limitation of appian since I have started practising appian.
When you upload a document in the form it is stored temporarily in the server and a temporary document id is created. When you submit the form it is stored in the targeted KC and document id is incremented.
I would like to correct something in your post, there cannot be fields in a CDT which are of type Document.
But I think you intend that there are some field that are of type integer and store the document id.

For achieving your requirement there are some ways which i would like to share.
You can have a buttonWidgetSubmit with name "Upload" and style "NORMAL" which upon clicking submits the form, goes into the process model (which helps the document getting upload into the targeted KC) and then comes back to the same form immediately through activity chaining.
There should be a XOR gate which should check that the submit action was from the Upload button not from actual submit button.
After the XOR check there should be a script task by which you can initialize the document id cdt field with the actual document id.
Now as you have the actual document id you can convert it into a document and perform a variety of validations.

Attaching some screenshot for your information.
Hope the information was helpful.

Upload Doc Form and process model.zip

I think this has been a behaviour/limitation of appian since I have started practising appian.
When you upload a document in the form it is stored temporarily in the server and a temporary document id is created. When you submit the form it is stored in the targeted KC and document id is incremented.
I would like to correct something in your post, there cannot be fields in a CDT which are of type Document.
But I think you intend that there are some field that are of type integer and store the document id.

For achieving your requirement there are some ways which i would like to share.
You can have a buttonWidgetSubmit with name "Upload" and style "NORMAL" which upon clicking submits the form, goes into the process model (which helps the document getting upload into the targeted KC) and then comes back to the same form immediately through activity chaining.
There should be a XOR gate which should check that the submit action was from the Upload button not from actual submit button.
After the XOR check there should be a script task by which you can initialize the document id cdt field with the actual document id.
Now as you have the actual document id you can convert it into a document and perform a variety of validations.

Attaching some screenshot for your information.
Hope the information was helpful.

Upload Doc Form and process model.zip