Why some PDF files are not selectable?

I try to get user to select a PDF file by this AS:-

set the_PDF to choose file with prompt "Select the PDF..." of type {"PDF "} without invisibles

But why not all PDF can be selected (some are grey out)?

Please advise. Tks!

because some .pdf files don’t have a ‘type code’.
if you don’t have to many, maybe this droplet mightl help…

on run
	tell me to display dialog "Please don't click on me." & return & "I'm an AppleScript droplet." & return & "Only drag and drop PDF files on me." buttons {"OK"} ¬
		default button 1 cancel button "OK" with icon stop
end run

property fileType : "PDF "
property creatorType : "CARO"
on open (theList)
	tell application "Finder"
		repeat with theItem in theList
			set file type of theItem to fileType
			set creator type of theItem to creatorType
		end repeat
	end tell
end open

Tom

Operating System: Mac OS X (10.5)