You may want to re-think your data architecture. A document library can contain custom columns for metadata, but if you have half a novel in a multi-line text field, that does not seem like a good approach.
Excel can easily store more than 800 characters in a cell, but it throws errors depending on the delivery method (formula vs. manual entry, one string vs. several concatenated strings, etc). Exporting the metadata of a library traverses from SharePoint to Excel and may well trigger an Excel cell limit, like 255 characters only.
Library columns are meant to add meta data to a document, like Business Area, Category, Client number, Expiry date, Shoe size, eye color, and in most cases such data will be a string much shorter than what you describe. I'm fairly certain that the developers of SharePoint never envisaged that a library column needs to house 800 or more characters.
Your question does not explain what you want to achieve: Why do you have such a ginormous text field in the doc lib metadata? And why do you need to export this to Excel?
If you edit your question and provide some more background about what you want to achieve rather than focusing on a single thing that does not work, we may be able to point you towards a better approach.
Edit: you did not really paint the bigger picture. How do you expect help if you don't reply to requests for clarification? As I see it, you have a multi line text field set to a "rich text" or even "enhanced rich text" as a column in the library.
Frankly, that is asking for trouble. Why would you want to have rich text in meta data? It just does not make sense.
Do yourself a favour and change the column to be plain text. You will lose the formatting, but then, by definition, metadata does not need formatting. If you struggle with that, you really need to revise your data architecture.