I am going to Print Boxes from a graphics file at 85 Line Screen. How can I generate a PDF that would be a good representation of how it would Print - for Proofing purpose?
You can create highly optimized PDFs for printing, but the concern is usually to get maximum quality from high-resolution four-color presses, where the details are critical.
For packaging print, as with most, everything lies with how the printer processes the PDF through their RIP (plate-maker or digital press controller), and they will set and adjust things like linescreen and color density to suit the press, the print stock, the job, etc. Those details are not the designer's concern, and rarely are.
For this kind of printing, almost any PDF of the correct size, orientation, etc. will do. If you have any questions about which standard to use, if any, ask the actual printer of the job. (Generic answers from here, the web or some other printer may or may not be accurate for them.) But again, for the relatively low print quality of packaging, PDF creation is generally non-demanding.
You should be able to simulate this in Photoshop.