I have recently tried to download 7.5-minute 1:24,000 quadrangles from the US National Map and noticed the only option offered on the original TNM download browser page is GeoTIFF. Simply clicking on the "download" will get you a GeoTIFF. Alarming, because my version 2.6.0.1 does not read that format and, of course, neither does Acrobat.
However, I clicked on the "Metadata" option and that opened a new browser page listing all the metadata for my topographic map. Halfway down the page is the "Downloads" section. It offers two choices: GeoPDF and GeoTIFF with appropriate links. I am happy to report clicking on the GeoPDF option gave me a 7.5-minute quadrangle that TransDEM 2.6.0.1 was able to georeference the way I have been accustomed.
The National Map Download page offered GeoPDF files when I attempted downloads in May 2018, so there was no problem at that time. Has anyone else had this experience? The work-around was fairly simple to use and I hope my sharing it will be helpful.
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