Completely new noob user here! Hi!

xnConvert was suggested to be by a colleague to change PDF's to TIFF for an archive project we're working on, I have it installed now and must not be setting it up right. I've chosen the PDF as the input, and TIFF as the output, then getting the error message that "it is not picture".
So really not sure what I'm doing wrong.
Can't use Adobe to save as TIFF as these are really big research posters we're saving so Adobe keeps telling me it's too large to output.

Any help would be greatly appreciated! Please and thanks!
Op