Good morning a409,
Welcome to the Apple Support Communities!
I understand you have some questions about backing up your PDF files, notes and voice memos. I'll do my best to clarify the difference between backing up and making copies of data. It is a good idea to have at least one backup, preferably two, of the data that is important to you. For the iPhone, the best practice is to have iCloud automatically back up your device every day, and to periodically make a second backup via iTunes. Similarly, on your PC, you will want to have a backup of your entire hard drive, usually to a (local) hard drive attached to it.
In general terms, you can't "see" the details of a backup file. When you go to iCloud, you will not see a backup folder. You can only restore items from a backup using the software on your iPhone or your computer.
How to back up your iPhone, iPad, and iPod touch
Creating a backup does not "move" files to a location either. If you don't already have a PDF file showing in the library of your iBooks app, then creating a backup isn't going to put it there.
So, the three-step process is to:
1) Import or add or create the data you want to the various apps or libraries or folders (PDF's, documents, photos, music, videos)
2) Have at least one, if not two, backups of all of your data in place (iCloud and/or iTunes backup), computer backup
3) Copy individual data or files that are important to you to another place (external local drive, iCloud Drive or other online storage)
For your PDF files, take a look at the resource:
Use iBooks with PDF documents on your iPhone, iPad, or iPod touch
Anything you create in iCloud (Notes, Contacts, Calendars, iCloud Photo Library) is NOT included in a backup of your device. This data is accessible to you anytime you sign in with your Apple ID and password and enable the feature in iCloud.
What does iCloud back up?
Therefore, it is important that you make a second copy of anything that is critical to you and that you archive your iCloud data.
Archive or make copies of the information you store in iCloud
Voice memos can by transferred to iTunes via a USB cable, and can also be shared to the Files app from your iPhone.
Sync your iPhone, iPad, or iPod touch with iTunes on your computer using USB
Use the Files app on your iPhone, iPad, and iPod touch
iPhone User Guide - Voice Memos
You can share individual voice memos and sync all your voice memos with the iTunes library on your computer.
Share a voice memo. Tap a saved voice memo, tap , then choose a sharing option, or save the memo to iCloud Drive.
Sync voice memos with iTunes. Connect iPhone to your computer. Open iTunes on your computer, then select iPhone near the top-left corner. Select Music in the sidebar, select Sync Music, select “Include voice memos,” then click Apply. Or, if iCloud Music Library is enabled on iPhone, select “Sync voice memos,” then click Apply.
When you delete a synced voice memo from iTunes, it stays on the device where it was recorded, but it’s deleted from any other iPhone or iPod touch you synced. If you delete a synced voice memo on iPhone, it’s copied back to iPhone the next time you sync with iTunes, but you can’t sync that copy back to iTunes a second time.
Voice memos synced from iPhone to your computer appear in the Music list and in the Voice Memos playlist in iTunes. Voice memos synced from your computer appear in the Voice Memos app on iPhone, but not in the Music app.
a409, I hope this general overview helps you with these broad, but important topics. Please reply to this post if you still need help saving and backing up your PDF files, notes and voice memos. The iPhone User Guide is a great place to start for any additional questions you may have.
Enjoy your day!