I'm a complete QGIS newbie so bear with me if I'm missing something obvious...
I'm unable to open GeoPDF's that I've downloaded from USGS's The National Map (TNM) site. This is the error that I get in the QGIS Log:
2019-09-24T12:44:03 CRITICAL Invalid Layer : GDAL provider Cannot open GDAL dataset /Users/jeff/Downloads/IL_Englewood_20120827_TM_geo.pdf: `/Users/jeff/Downloads/IL_Englewood_20120827_TM_geo.pdf' not recognized as a supported file format. Raster layer Provider is not valid (provider: gdal, URI: /Users/jeff/Downloads/IL_Englewood_20120827_TM_geo.pdf
When I run gdalinfo on the file I get this:
gdalinfo ~/Downloads/IL_Englewood_20120827_TM_geo.pdf ERROR 4: `/Users/jeff/Downloads/IL_Englewood_20120827_TM_geo.pdf' not recognized as a supported file format. gdalinfo failed - unable to open '/Users/jeff/Downloads/IL_Englewood_20120827_TM_geo.pdf'.
From everything I've seen online GeoPDFs should be fully supported in QGIS 3.8.3 and gdal 2.4.1. I can also open the file in the standard Apple Preview app without trouble.