It is not possible to fix the display order of manuals when a viewer opens a folder.
However, you can adjust the display order when the viewer sorts the manuals by name. Add numbers like '01, 02, 03' or symbols like 'a. b. c.' to the beginning of the manual titles.
If you want to have viewers read the manuals in the desired order, consider using the Portal Page feature or the Training feature.
Related articles:
Configure Display Items and Sort Order for Manuals (Web Browser)
https://teachme.jp/8/manuals/9552194
What is the character priority order when sorting manuals by name?
https://help.teachme.jp/hc/en-us/articles/4416535551897
What is the Portal Pages function?
https://help.teachme.jp/hc/en-us/articles/21838465421593
What is The training feature?
https://help.teachme.jp/hc/en-us/articles/7441713539993