rename a PDF file using an internal info with Automator

This will replace the original PDF name with the captured text string from the PDF, and then rename the original PDF with that string (e.g. foo.pdf becomes ddt_2345.pdf. Tested with macOS 11.4.

However, this does not account for finding that formatted document number when we don't know it in advance. I will need to write a different script that dynamically finds the PDF document number and uses that for document renaming. Will the document number always be on the first page of the PDF?

use framework "Foundation"
use framework "PDFKit"
use AppleScript version "2.4" -- Yosemite or later
use scripting additions

property NSString : a reference to current application's NSString
property NSURL : a reference to current application's NSURL
property PDFDocument : a reference to current application's PDFDocument
property NSLiteralSearch : a reference to current application's NSLiteralSearch
property NSCaseInsensitiveSearch : a reference to current application's NSCaseInsensitiveSearch

# how to bitwise OR integer items like n | m
set search_mask to (NSLiteralSearch as integer) + (NSCaseInsensitiveSearch as integer)
set search_str to "ddt_2345"

set pdf_file to POSIX path of (choose file of type {"com.adobe.pdf"} default location (path to desktop)) as text

set pdf_url to NSURL's fileURLWithPath:pdf_file
set pdf to PDFDocument's alloc()'s initWithURL:pdf_url
set selection_array to pdf's findString:search_str withOptions:search_mask
set the_text to (selection_array's objectAtIndex:0)'s |string|()
if not (the_text = "" or the_text = missing value) = true then
	display dialog "Captured text is: " & the_text as text with title "Capture Results"
	tell application "Finder" to set the name of ((POSIX file pdf_file) as alias) to my change_pdf_name(the_text, ".pdf")
else
	display dialog "Captured text is not found"
end if
return

on change_pdf_name(newtxt, ext)
	return ((NSString's stringWithString:newtxt)'s stringByAppendingString:ext) as text
end change_pdf_name

On macOS 11.4, this Automator Folder Action worked and moved the dropped (and renamed) files out of the drop folder into the Desktop Metadata folder. I just copied and pasted the most recent (2021-07-11) code above into a blank Automator Run AppleScript action, and saved that folder action with a new name, but with the same drop folder. I then dropped my four PDF test files onto the drop folder and all were subsequently renamed and moved to the Desktop Metadata folder.

The search pattern that I am applying on the first page of the PDF allows for none, one, or more white-space characters to precede the Document Number at the beginning of the text line. Any other characters before the document number and the same line will fail the capture and document rename.

I suggest you start over with a fresh Automator Folder Action and follow my steps carefully.

A proper folder action has no "Ottiemi elementi del Finder specificati" action in it. Just the Run AppleScript action with the code that I provided. You specify the drop folder and save the Folder Action. These are intended for you to drag and drop files onto the drop folder, from outside of the drop folder, and that behavior triggers the folder action code.

Remove the first action, save the Folder Action, and then drag/drop that same PDF with a document number from another folder location onto the designated Folder Action folder.