As you are browsing the web, you may come across a PDF file you want to save for later viewing. Let’s examine how to do this.
This if for Apple iPhone and iPad

While viewing the PDF file in Safari, tap on the Share button. The Share button resembles a rectangle with an up arrow. On an iPhone, the button will be at the bottom of the screen. For an iPad, the Share button is located just to the right of the browser’s address bar. See following image for the iPhone location:

Ways to Save Your File
After tapping the Share button, you will see a list of ways to share or save the file, including sending the file to others by a text message or email. Refer to below image:

Swipe down the list to find the app you want to open with. See following image:

Use Built-In Apps
You may want to chose the built-in apps like Books or Files. If you want to use Books, the file will be available in the Books app. If you chose Files, then chose your save location.
Use Outside Services
Some people prefer to save their file to an outside service like Dropbox, Google Drive or Microsoft OneDrive. The choice is up to you.
My Example
For my example, I am saving the file using the Files option to the PDF folder on the iCloud Drive. Refer to below image:

Later, if you chose the Files app, you will see the PDF file you just downloaded; same for the Books app. See following image on my choosing the Files app:
Chosing the Files App

As you have seen, you have a lot of choices, thanks to Apple engineers.
I Would Like to Hear from You
Please feel free to leave a comment. I would love hearing from you. Do you have a computer or smart device tech question? I will do my best to answer your inquiry. Please mention the device, app and version that you are using. To help me out, you can send screenshots of your data related to your question.