Hi Sébastien,
If i understand you correct, you have 2 questions:
How to get the file in a folder with a certain extension
How to open that file (PDF) without interfacting with Adobe Reader.
To get the File(s) in a folder with a PDF extension, you can use the object Utility - File management with as action Get Files.
As Folder, you give the path to the folder with your file(s).
as Pattern CSV you give in your case ""*.pdf""
Then to open the PDF without interfacting with Adobe Reader, you can use the object Utility - Environment with as action Start Process to open the file
(As input Parameters; Application the full path to Adobe acrobat, as Argument ""/s "" & the full path to your PDF file - Which you got at step 1)
the /s prevents Adobe Acrobat from opening the Splash screen, so you'll see the file right away.
Chances are that we're working for the same company, so try to Ping me on our company communicator if you have any questions.