Hello CParvi,
Thank you for contacting Apple Support Community! I see you are having issues opening you pdf saved files in your Files application on your iPhone 7 Plus, It is important to be able to easily access your files seamlessly as intended.
To be able to open your files from the Files application specifically the ones saved on iCloud Drive you need a good internet connection, if you are trying to access them using your cellular data connection it will be affected with the speed and strength of your cellular connection, using Wi-Fi with iCloud files is faster, or if you don't want to use the internet to retrieve your files daily you can always store your pdfs on your local iPhone memory if your storage is efficient for that, How to check the storage on your iPhone, iPad, and iPod touch
For the Files application freezing or not responding you can check this article, If an app unexpectedly quits, stops responding, or won’t open.
If force quit or restart the device frequently doesn't help the freezing of the app then following the steps for hard restarting your device will be your next step, following this part " Press and hold both the Top (or Side) button and the Volume Down buttons until you see the Apple logo." in the article If your iPhone, iPad, or iPod touch won't turn on or is frozen.
The freezing of the application while trying to retrieve data from iCloud might be related with how much storage you have left on iCloud, Manage your iCloud storage
This article shows more information about using the Files Application, Use the Files app on your iPhone, iPad, or iPod touch
To be able to access the files you stored in the iCloud Drive from other devices this article has guidelines steps, Access iCloud Drive files from your iPhone and other devices
You can also delete the application and re-download it again from the App Store if the freezing problem was not resolved, How to delete apps on your iPhone, iPad, and iPod touch
Take Care!