Just replace the file/folder path placeholder with the file/folder path to ignore.
xattr -w com.dropbox.ignored 1 /Users/yourname/Dropbox/YourFileName.pdf
attr -s com.dropbox.ignored -V 1 /Users/yourname/Dropbox/YourFileName.pdf
Set-Content -Path 'C:\Users\yourname\Dropbox\YourFileName.pdf' -Stream com.dropbox.ignored -Value 1
Symlinks and/or having more than 300,000 files synced to one computer can cause Dropbox performance to decline.
This is a soft limit, and depends highly on the hardware specifications of the computer running the Dropbox application. If you think this might be part of the problem, I'd recommend using Selective Sync (https://www.dropbox.com/help/175) to sync only the folders you need to work with to your computer.