If an email is queuing and then bounces or reports back as a failure or bounce notification, then it may be that the email address it has tried to send to is incorrect.
However, if the emails are queuing and simply not sending, then it will usually mean that they have certain attachments that are too large (eg: a single attachment size is 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, in the Web Portal, by going to Admin > View Email Log > Forms Email log (if the unsent email is for a form) or CRM Email Log (if the unsent email is an invoice or other CRM communication).