Hope you are doing well and sorry for the trouble. If I understood correctly, the test in the PDF sent to you disappears when you save it to your computer, correct?
It is likely that the font used for the disappearing text is either not embedded, /or not present on your machine. This does not allow Acrobat to embed the font into the extracted document, and that's why it is missing, and does not display.
Please try the following preference setting and see if that helps:
Go to Edit (Windows), Adobe Acrobat (Mac) > Preferences > Page Display > Under 'Rendering' select 'Use Local Fonts' > Click OK and reboot the machine and check.
Also, go to Tools > Print Production > Preflight > Under 'Profiles' > Select 'List potential font problem' > Click On Analyse
If the required font is available on the OS, once you go to Preflight select single fixups and then select Embed font and click on fix as shown below and see if that helps.