When RC (or any mail client) encodes an attachment, there is a fair amount of overhead added. Your 2MB upload limit is probably not sufficient to allow a 1.4MB attachment. Try increasing it to 4MB.
This tricks people who set *attachment* size limits, but shouldn't have much of an effect on the POST or upload sizes. I believe the only overhead is for the multipart/form-data encoding and for the HTTP headers, both of which are relatively small.
I just checked that Roundcube does upload a PDF as multipart/form-data (and not as, say, application/x-www-form-urlencoded). Maybe if this remains a mystery the OP can see what it does for his file; I used the Firefox Live HTTP Headers extension.
My file is actually a PDF too.