For me, the manual starts the pdf reader after downloading in just a few seconds from either your server or the Garmin USA server. But I'm on an 80mbps fiber link. My Ping times are also low.
Another thing to try:
Right-click, select Save Target As and see if both locations save the file easily. That could indicate a problem with the pdf reader for your browser. But it wouldn't explain why the Garmin server works and ZumoUserForums doesn't
See how responsive your link to each server is:
PS C:\Users\dave817> ping
www.zumouserforums.co.uk
Pinging zumouserforums.co.uk [87.76.30.87] with 32 bytes of data:
Reply from 87.76.30.87: bytes=32 time=116ms TTL=49
Reply from 87.76.30.87: bytes=32 time=122ms TTL=49
Reply from 87.76.30.87: bytes=32 time=115ms TTL=49
Reply from 87.76.30.87: bytes=32 time=114ms TTL=49
Ping statistics for 87.76.30.87:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 114ms, Maximum = 122ms, Average = 116ms
///////////////////////////////////////////////////////
PS C:\Users\dave817> ping
www.garmin.com
Pinging e3664.b.akamaiedge.net [23.202.233.25] with 32 bytes of data:
Reply from 23.202.233.25: bytes=32 time=10ms TTL=56
Reply from 23.202.233.25: bytes=32 time=11ms TTL=56
Reply from 23.202.233.25: bytes=32 time=10ms TTL=56
Reply from 23.202.233.25: bytes=32 time=10ms TTL=56
Ping statistics for 23.202.233.25:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 10ms, Maximum = 11ms, Average = 10ms
PS C:\Users\dave817>