I have not tried it with PDFs myself I always just make it an in image file. The expression I use is [Switch] & ".jpg".
I place all the images in the same folder & make them the same file type. The images must have the same name they have in the database switch field. So if the database filed that determines the switch is called Cedar Creek, the image must be named Cedar Creek.
Just import one of the images. Right click on the box & select Edit Expression. Enter the expression above. The part of the expression I called Switch should be whatever the field name is in the database & between the quotation marks should be the extension of the image format you are using.
If it still does not work after this you may have to link the image box to the folder they are store in. to do this just click on the box & in the Properties Tab, under Content you should see Image Folder. Click on the right side of it & browse to your image folder.
I hope this helps & was not to confusing. I know there is another way of setting up image switches but this has always worked for me.
Good Luck!