Sorry for the delay in response.
As you are unable to open a downloaded file on your phone, could you check if the file you are trying to open is a valid pdf file. Try opening another pdf file and see if you are still receiving this message.
Also, if possible, try opening the same file in Adobe Reader desktop application and see if that works.
You may visit the following link which discusses the similar issue:
Hi unable to open d file
Unable to open file asking password
1 person found this helpful
Ask the file's owner for the password