It seems to matter that your extensions are upper-cased:

>> Dir['*.tif']
=> ["1_lower.tif", "1_upper.TIF", "2_lower.tif", "2_upper.TIF"]

Running Roberts code gives me this:

1_lower.tif   1_lower.pdf
I got you
1_upper.TIF   1_upper.TIF.pdf
2_lower.tif   2_lower.pdf
I got you
2_upper.TIF   2_upper.TIF.pdf

Very interesting.