I found that you can use bombs to find two of the four lost pages. One on level 6 and one on level 24. Simply take the bomb to the faded bricks and use it there. In order to complete 100%, we need to work as a community to find the last two pages. Let us know if you find them :))

There are only 2 pages for 100% Official response from the dev:

Thank you for playing Infernum Peak!

The last two pages are still unrealized. At the moment, work on Infernum Peak is temporarily suspended. Unfortunately, I can't give a date for the resumption of work on Infernum Peak.