Hello @FUYÉERÉT ACCOUNT ,

Please following steps to use custom fonts with Office Online Server (OOS):

1. Font files must be ".otf" files. The .ttf files don't work with OOS.
2. To apply custom fonts in web apps, install the ".otf" file on your local computers and every OOS server in the farm.
3. The font file must be installed by an administrator, and it must be installed by right-clicking the font file and clicking Install. Note: You may not just drag the font file into the fonts folder.
4. Fonts will not appear in the dropdown list of fonts, but the Font is it possible to be searched using the searchbox. Note: It has to be spelled exactly as the name of the Font. For example, if the actual custom font name is "Fonts", and the user searches for "Font", the custom won't be used.

By design, We can not get rid of the limitation of the size of open files online in Office Online Server.

File Size Limitation. If you have large files (>10MB in size), you will be prompted to open the file in the native (desktop version) of the Office. For example, below is a message you get when you try to open an Excel workbook larger than 10MB in size.

Read more information: Advantages and Limitations of using Office Online in SharePoint and OneDrive

Thanks, Echo Du

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