Jielle,
Let's take this one step at a time. What print jobs are you creating with which programs? Are you using a full version of Acrobat which includes the pdf writer "printer" to generate a pdf file? You said you were "making" a pdf file and then using Acrobat 5 to open it and print it. Is this file being generated by your invoice software, then you are using the PDF writer to print the results to a file which you are then opening with Acrobat 5 to print?
The reason for my asking all this is I have just been looking at an older version of Acrobat writer on my computer here. I cannot get it to default to anything but letter (I'm using a US OS so the default is different). The instructions given before about using the option key when clicking "OK" in page setup seems to work fine when I am using the laserwriter printer, but these new defaults do not transfer over to the pdf writer when I choose it as a new printer (and yes, I am quitting applications between changing printers). The option key doesn't seem to work the way it does with the laserwriter printer. I suspect this is something to do with Acrobat and I don't know if there's anything we can do.
If you are not generating the file the way I said then I am puzzled by the default not changing, although maybe if you are using a third party printer driver it may not be configured to let you do that. I only have the Apple laserwriter one on my computer and it clearly lets me change the default when I hold down the option key when clicking OK. If you have an application that is already open then you must quit it, but after that it keeps whatever default you have set up. I was mostly working with simple applications to test this but this included Acrobat Reader 4 and it would always open to whatever default setting I had configured before.
So:
Open an application, and a document if necessary.
Select File/Page Setup
Change the default to letter
Hold down the option key at the same time as you click "OK" The computer will ask you if you want to change your defaults, and you say yes.
If you close the application or open any new ones and look in page setup it should show the new default. If you have any applications already open they will not show the changes until you quit and reopen them.
Cheers,