The space shouldn’t cause any problems. In your Start Process, your argument (or filename) should be embedded in quotes.
For example, pdf (1).pdf will need to be “pdf (1).pdf” and since the whole thing needs to be a string also you need to double up on quotes.
If you were to output your argument in a message box or write line, it should show the quotations also, just like if you were opening it in commandline.
You can alternatively use the Open Application activity to open the file.
It might also be useful to know that you can get your default Adobe Reader from the registry with this line: