PDF file

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

I want to upload a pdf file(book) so I drag file upload from interface components but I don't understand what should fill in the target folder and it will save it in DB by the CDT of which data type?

And this target folder should call a constant of document type or what type to save in it? as in the attachment image below?

the document tells you:

1. create a folder in Appian Designer e.g. XXX_FOLDER (where 'XXX' is your application prefix)
2. create a constant of type folder and make it refer to the above folder e.g. XXX_FOLDER_<folderName> where 'XXX' is your application prefix and <folderName> is the name of the folder you created
3. set 'target' in your code to cons!XXX_FOLDER_<folderName>

the document tells you:

1. create a folder in Appian Designer e.g. XXX_FOLDER (where 'XXX' is your application prefix)
2. create a constant of type folder and make it refer to the above folder e.g. XXX_FOLDER_<folderName> where 'XXX' is your application prefix and <folderName> is the name of the folder you created
3. set 'target' in your code to cons!XXX_FOLDER_<folderName>

can you tell me about the constant (Type and value ) 

https://docs.appian.com/suite/help/21.4/Constants.html

The public documentation that Appian provides is pretty self-explanatory on this. The concept of a constant isn't unique to Appian, it's pretty much constant (excuse the pun!) across all modern programming languages.