Highlight a PDF, control click and choose "get info>Open with. Choose PReview and click "change all".
What do you want to open PDF documents with, and what do you mean by background if I may ask?
It is on or it is off and in background to me means that it is running while you run some other application on top of it, or hide it.
Now if you are not wanting it to run except when you open it then ~graffiti has the answer.
Choose the Application that you want to open PDF files by default and choose Change All while in the Get Info window of one PDF document and that should get it done.
You can verify this by going to a different folder and Get Info on another PDF document.
I keep all my PDF files in a master PDF folder, and then in various, applicable, sub-folders.
Thanks for your answer. You hit the nail right on the top. It was just what I wanted to know.
I was not so clear in my description, I agree. For PDF files I use Preview and Adobe 9.4.0 The first is 13.2MB, the latter 234MB. I run Mac OS 10.4.11 on a PowerPC G4 and my RAM is limited, so for smaller and simpel documents I open Preview and Reader for the advanced ones. For BackUp purposes I have indeed all .pdf files in special folders to keep them outside the BackUp proces. Does this make sense?