Can you share the font you used and a screenshot of the PDF before and after the download? Is it a font from Adobe or from somewhere else? Did you use the font in a video or image template? Did you try to download the project with a different browser?
Can you confirm the browser and the OS version on your device?
I have been experiencing the same issue while downloading the Hindi text to PDF from Adobe Express. The text goes scrambled. After a few workarounds, I figured out a way to fix this.
1. Create your design in Adobe Express anyway because it's the best.
2. Save it as a PDF.
3. Upload the PDF document to Canva.
4. Upload the Hindi font to Canva if it's unavailable.
5. Edit/modify the Hindi text.
6. Save the document as PDF.
7. You have done it. Congratulations!
I don't know how long Adobe will take to fix this issue, but people who invest their time in designing can't wait until they bring up a resolution.