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a!fileUploadField + a!documentViewerField

Do you have any specific question? There is documentation and tutorials available.

That's when we need to view the document not when we need to get it and save it in the application. How we can get it by uploading a file in the upload file field then saving it as a document in a folder in the app?

I am not sure I understand that. Please elaborate.

What do you want to achieve? What did you try? What is going wrong?

You'll need to think of the solution in two parts:

1. uploading the document - for this you'll need a Process that you can run on demand. This can be an "Action" (if your documents don't need to be associated with any other business entity); or as a "Record Action" where the document does need to be associated with a business entity. The process you run will need to present a User Input Task to the user which will have an interface that contains the a!fielUploadField that Stefan mentioned. Either way you'll probably want a database entity to hold some "Metadata" about the document - data that you can't natively get from the file properties
2. view the document - you need to be able to navigate to a context where you can view the document e.g. a Record view. There you'll need an Interface that contains - as Stefan says - the a!documentViewerField

You'll need to think of the solution in two parts:

1. uploading the document - for this you'll need a Process that you can run on demand. This can be an "Action" (if your documents don't need to be associated with any other business entity); or as a "Record Action" where the document does need to be associated with a business entity. The process you run will need to present a User Input Task to the user which will have an interface that contains the a!fielUploadField that Stefan mentioned. Either way you'll probably want a database entity to hold some "Metadata" about the document - data that you can't natively get from the file properties
2. view the document - you need to be able to navigate to a context where you can view the document e.g. a Record view. There you'll need an Interface that contains - as Stefan says - the a!documentViewerField