We haven't received similar issues reported. To better isolate the issue, you might need to have your client to switch to a different browser. The browser’s overloaded cache could be the reason why they're unable to receive the file.
Your client can use these keyboard shortcuts depending on the browser they're using:
Google Chrome: press Ctrl + Shift + N
Mozilla Firefox: press Ctrl + Shift + P
Internet Explorer: press Ctrl + Shift + P
Safari: press Command + Shift + N
Once your client logged into incognito windows, you can try to send the quotation PDF file. If it works, have them clear the browser's cache. This removes the history or log of sites so you can start with a clean slate.