A few puzzlements for me.

I added a new chapter as a folder and called it Chapter 7A and added a new scene. The folder sits between the folder for Chapter 7 and Chapter 8

Chapter 7
Chapter 7A
Chapter 8

I typed a few paragraphs and saved it. Out of curiosity, I compiled the document, yet chapter 7A does not appear in the PDF pages. Any ideas?

Probably Chapter 7A is not included in the list of contents on the main Compile screen. See Section 23.4.1 in the Scrivener manual for more information.