Get one long tiff from a multipage-PDF with these two simple commands

Every once in a while I receive a well log or mud log from a client in multipage-pdf form. In other words, rather than a single, very tall raster image (in tiff format, for example), the image has been cut up into many, say, 8.5x11-inch or A4 pages and saved in pdf format. Instead, I often want a file as a single, long page in tiff format for use in Petra or to send off to a digitizer.