PDF files not displaying when...........

My daughter has been using an iPhone for 5-7 years or more now. When I do a backup, it is 'assumed' that everything is backup like it says. When I do a restore from backup... the iBooks, photos, etc DID NOT COME BACK. Some data such as school schedules, Homework, etc etc etc.. comes from many many many sources. I have talked to a blind specialist but he beat around the bush.... etc.. it can come from a multitude of sources, etc. yes I know. When I do a backup and restore on a PC (using Reflect, GHOST, Acronis Backup-Restore, Windows 10 phone, Android phone) or the devices I listed. Everything comes back. No buts or if's or sources where it comes from...... it darn comes back that is all!!. Some data (PDF, Photos, Music - can be instructions on a procedure to a blind person (I am also member of the blind association for the Blind in Tennessee)is crucial and even possibly life threatening... if a back DOES not do what it says why use iTunes or even the point of doing a backup. A modern PC using Acronis or any software can backup... 1000s of terabytes why a 32 or 64gb phone cannot even be decently back up and restore. it is the most ridiculous thing I have ever heard in my life. I am so disappointed in Apple. If I I had a choice I will never use Apple iTunes. As a lead qa I know this can be done easily by Apple and not giving so much lame excuses.

Phillip K