Q:

Autostart Python script doesn't work

I am trying to get my Raspberry Pi to open a terminal and run a Python script when it boots up.  I have followed the instructions of some guides and have so far been unsuccessful. 
My steps
I have made a Python file called testfile.py located in /home/pi/Documents/testfile.py
The contents are as follows:
print('hello')
input('press any key to exit')

I have added a file called testfile.desktop to /home/pi/.config/autostart/testfile.desktop. 
The contents are as follows:
[Desktop Entry]
Type=Application
Name=testfile
Hidden=false
NoDisplay=false
Terminal=true
X-GNOME-Autostart-enabled=true
Exec=/usr/bin/python3 /home/pi/Documents/testfile.py

When I double click this file, the terminal opens and the Python script runs as expected, but when I reboot the Raspberry Pi the script doesn't run. 

A:

I duplicated what you did using an up-to-date Raspbian Buster in a Pi 4 B.  (I wish all questions were as clear and complete as yours!) 
Double-clicking the file on my machine brought up the thonny IDE.   (Note:  The 'any' key doesn't work because input() needs an enter to complete.)  On reboot, I observed the same result you did.   However, installing xterm and changing the Exec line to
Exec=xterm -hold -e '/usr/bin/python3 /home/pi/Documents/testfile.py'
causes an xterm window to open and the Python program to run.   I was following these debugging instructions. 
So, there is some interaction I don't understand between autostart, Python, and terminal.   Perhaps you could try a Python program that leaves some evidence in the file system, such as writing a tiny output file.   
Edit: See this answer which appears to be more useful than mine.