It is the server softwear that regulates what can be uploaded.
IT is quite normal for forums not to allow big files to be uploaded.
Remember what you see is not what the server saves.
Also just the same as you the forum owner has to pay for uploading & downloading.
What you are uploading, it is downloading & visa Versa.
This forum is almost unique in that it allows direct uploading of photos.
Most only load a refference URL to a third party image hosting site, so when that goes so do the images.
Finally there are copyright problems .
Stihl ( prime example ) actually sues people who load their IPL &/or service manuals on line and I am sure other makers do the same.
Finally server hosts charge the operator for volume of data uploaded, downloaded & saved onto hard drives .
large manuals like the JD ones with lots of photos are very large files.