Usually when emails have certain attachments that are too large such as 25mb, the size of the file is too large and much larger then other attachments. This means the larger the file, the longer it will take for the email service to process and send to your client. 

Reducing the file size of this attachment will improve email speed and deliverability in the future. 

A good rule of thumb is to keep the total size of all files attached to any email to less than 10mb as some inboxes will not accept emails larger than 10mb. 

Once the reduced size of that specific PDF, remove the large file attached to the smaller file, the emails should send through faster. It is recommended to getting the file size as close to the size of your first file as you can. 

You can check that the emails are being sent, Web Portal > Tools > Email logs.