The EB Garamond font family I have (from Google Fonts) contains variable fonts and static fonts. Any support for variable fonts in LibreOffice is accidental. It might be best to check that you have the static fonts installed rather than the variable font.
The Readme says
If your app fully supports variable fonts, you can now pick intermediate styles that aren’t available as static fonts. Not all apps support variable fonts, and in those cases you can use the static font files for EB Garamond:
As an experiment, I installed the variable non-italic version of EB Garamond and was surprised to see that it worked, showing regular, ExtraBold, Semibold, & Medium. The pdf I exported with those variations, sadly, contained only the regular version but did open in Adobe reader and a couple of other pdf readers as well as Firefox. It might not be the reason others cannot open your pdfs but it is worth checking.
Later: Out of curiosity, using text in different weights of variable EB Garamond, I experimented with Print to PDF (Adobe Acrobat CS5 and Bullzip) in both cases the text looked correct weights but all text was rendered as curves