How do I transfer document files from my PC to my iPhone?

Use iCloud Drive, see this Apple articles below:

Set up iCloud Drive

Set up iCloud Drive - Apple Support                                                  

Access and manage your files in iCloud Drive

Access and manage your files in iCloud Drive - Apple Support

Transfer files between iPhone and your computer

Intro to transferring files between your iPhone and computer - Apple Support (BN)

Share files and folders in iCloud Drive on iPhone

Share files and folders in iCloud Drive on iPhone – Apple Support (UK)

Use iTunes to share files between your computer and your iOS or iPadOS device

Use iTunes to share files between your computer and your iOS or iPadOS device - Apple Support