I am using someone else's text from another forum but I am experiencing the same issue with Adobe Acrobat Pro DC, Version 2021.001.20142:
This is the issue at hand: "I saved a PDF (text) that I had been editing with highlights. When the PDF was saved all of the text disappeared, leaving blank white pages with my markups. This has happened multiple times. I would like to [...] recover the text of the PDF while maintaining the highlights [...]."
Edit and convert PDFs, General troubleshooting, How to
Hope you are doing well and sorry for the trouble. As described the text from the PDF disappeared when you try to save it after highlighting.
Is this a behavior with a particular PDF file or with all the PDFs? Please try with a different PDF file and check.
If the PDF file is stored on a shared network/drive please download and save it to your computer first and check if that helps.
Also, please make sure you have the latest version 21.01.20145 installed Go to Help > Check for Updates and reboot the computer once.
Adobe Acrobat/Reader DC automatically saves the document changes to the temporary file if you have the save settings enabled. Go to Edit (Win), Adobe Acrobat (Mac) > Preferences > Documents > Under Save settings Select 'Save document changes to temporary file after every X minute'
This has happened to me throughout the year. It is not every pdf document but definitely a few. They are all saved on my hard drive. It happens randomly. I have got the latest version and it automatically saves temporary documents every 5 minutes.
I have been creating comment summaries (text pages on the left are blank showing highlights, the comments on the right remain) to not lose all my work but it is still a hassle because I have to download the original pdf document and compare it with my notes.