If you have MS Access 2k7 updated to Service Pack 2, then you should have the save as PDF Add-in already. Otherwise you can download just the Add-in from:
http://www.microsoft.com/downloads/details.aspx?FamilyID=f1fc413c-6d89-4f15-991b-63b07ba5f2e5&displaylang=en
To save your report as a PDF open the report in design view. Select or create your button. Right click on it and go to properties, select the event tab. In the on click event, click the "..." Select "Code Builder". Enter the following:
Private Sub YourButtonName_Click()
On Error GoTo Err_YourButtonName_Click
DoCmd.OutputTo acReport, "YourReportName", acFormatPDF
Exit_YourButtonName_Click:
Exit Sub
Err_YourButtonName_Click:
MsgBox Err.Description
Resume Exit_YourButtonName_Click
End Sub
Replacing YourButtonName with the name of your button and;
YourReportName with the name of your report.
In the properties of the report itself on the format tab, you will also want to change the "Default View" to "Report View". And on the properties for the button, on the format tab, change "Display When" to "Screen Only" so that the button images do not get saved to the PDF.
There are other things you can do such as specify the name and location of the PDF file, but i suspect that this will get you started.
Hope this helps!