Hi
I need a module I can define the fields I need in it, so visitors can enter the information
then admin can see the submitted data from users see their data and export them to PDF

is there such that module ?

I think I need a few more details.

The Form and List module can be used to create a form and use data from the form can be saved in a data table and/or emailed to an administrator. Dynamic Forms and Form Master are commercial modules that also do this (and perhaps more). There are modules that will create PDF files (from Onyak Tech, I think).

I want after defining the fields I need, the user can add items like UDT
but when admin see the items added he can export them or an item to PDF

Maybe UDT for the form and data collection and Onyak's product for creating the PDF reports.

and how I could connect between them ?
can you give me an example

If you create a form with the Form & List module, then the data will be stored in the database table. You could also do this with either Dynamic Forms or FormMaster. It would be "easier" to do this with either Dynamic Forms or Formmaster because you have more control over the table that is used for data storage.

You will then need to construct a SQL query to extract the data from that table. You would do that in the Onyak module.