You've posted here in the Living Atlas Community, this question might be a better fit in the ArcGIS Pro or ArcGIS Online Community depending on what you're looking to do.
ArcGIS Living Atlas does have a selection of content for Vietnam:
https://livingatlas.arcgis.com/en/browse/?q=vietnam#d=2&q=vietnam&rgnCode=VN
In addition, Living Atlas basemaps such as World Topographic, World Imagery, and the others do have global coverage.
If you are specifically after those files stored in the UT Austin archives:
https://maps.lib.utexas.edu/maps/topo/vietnam/
I downloaded one of those to experiment. While you can add the PDF file as a document in ArcGIS Online, I am assuming that you want to add the topos as layers in a map. To do that, you'll need to use ArcGIS Pro to convert GeoPDF to GeoTIFF, then publish it from there. If Pro is your goal, then the conversion from GeoPDF to GeoTIFF should be what you want:
https://pro.arcgis.com/en/pro-app/latest/tool-reference/conversion/pdf-to-tiff.htm
After successful completion of the conversion, you would need to publish it as a service for use in a web map.
I gave this a quick try and Pro complained that there was no spatial reference for the GeoPDF I downloaded from the UT site. While it successfully converted the file to TIF, it did not have a spatial reference.
Next I gave it a quick go in a public domain GeoPDF converter. While it successfully converted to GeoTIFF, there were obvious issues in the resultant file, making me suspicious of the source files.
Were it me, I would double check the topo archive to make sure they are proper GeoPDF files if your goal is to publish these a map services for use in web maps.
Others here may be able to offer other advice..
