Best way to transfer & KEEP PDF files on iPhone 15

iPhone 15 Pro Max, iOS 17.2

I want to be able to keep some PDF files on my iPhone as I had done with Android.

I have an iMac & thought t would be easy to transfer files back & forth w/o having to use iCloud.

Much like I can transfer files to & from a flash drive, via USB.

I wanted to use the iPhone Blue “Files” folder which was installed by default)

The best way I have found (so far) to move a PDF file from Mac to iPhone is:

Connect iP-15 to Mac

Open my iPhone folder in Finder window (It appears in Left side panel under “Locations”)

Click “Files” heading

Drag the PDF file to “Acrobat” (I had installed the Adobe Acrobat app on iP-15)

Disconnect iPhone from Mac (Note: I always eject it 1st but, not sure really necessary)

Open Acrobat app on iPhone

At bottom of screen; Home, Files, Shared & Search

Tap Files

Heading: “Locations” with 9 items listed, tap “On this iPhone”

In the list, I see my newly transferred PDF file.

I tap to open it.

Once I do that, I close Adobe app & go back to iPhone Home screen (2nd page) & tap the Blue “Files” folder.

On bottom of screen; Recents, Shared & Browse.

Tap Recents & Voila! (Finally) I see my newly transferred PDF file!

(BTW, it never appears in the “Browse” section)

Here is my problem:

After days or weeks, that file, as well as all my previously transferred PDF files are GONE!

Through trial & error, I discovered I can go back to Acrobat & find the missing files & quickly open & close each one & then they again appear in the iPhone Blue Files folder under “Recents.”

I suspect they disappeared because after a certain amount of time, they were no longer recent.

There must be a better way!

Thanks for any help.