Solution

1. Use Pushbutton ... process=... to add a button that starts an InputScript
   
2. In the InputScript, use the Start command to display the PDF file

Example
We add a button in the initial screen of VA01 that makes work instructions available to the user:

GuiXT Script

Pushbutton (toolbar) "@5E@Instructions" process="display_pdf.txt"
     using PDF = "va01.instructions.pdf"

InputScript "display_pdf.txt"

Parameter PDF // PDF file name

// display .pdf file
Start &U[PDF]

// return without further actions
Return

Remarks

You may specify an absolute path for the PDF, a web URL, a file from SAP Web or Mime Repository, or a relative path which then refers to the script directory.

Components
InputAssistant