how user can enter only one pdf or one file not list like this is there any built-in item do this ?

Hello, 

Kindly provide more details about current behaviour and expected result so we could assist further. 

Oleksandra,

ok for example if I want user to enter his HR letter ok I will add input field which is attachment in the form page which will allow him to upload files but now we have a problem that user can upload more than one file but we need only one HR letter not list of HR letters 

Abanoub Saleh,

You can add a process which deletes from parent record all files except the last loaded one (Id <> Id of the last loaded file) in case of the new file was loaded in the object event. It's the simplest solution.