Including attachments in email without first communicating to the recipient that you will be doing so can really get your email into trouble. This is true for all types of email, including transactional email, one to one email, email marketing, and other bulk email.
This holds true across the spectrum, for all sorts of business and even personal email, and for all sorts of attachments, such as .doc files, .xml files, and even, yes, PDFs. This may surprise you, as so many businesses and email senders have started just automatically sending things such as receipts as a PDF. And it is true that many of them make it to the inbox, primarily email senders who already have an established online presence and email reputation, for example a doctor’s office. Then again, by the time you are getting a receipt from a doctor’s office you a) have probably already exchanged or at least received email from them, and b) they have likely asked you whether you’d like your receipt handed to you, or emailed to you, or put in the USPS mail.
Another place where attachments (primarily PDFs) have become increasingly common is as “freebies” that a business will send in exchange for signing up for their mailing list. Heck, we do that ourselves (see that green button in the lower right-hand corner? Click it to see what we mean). These examples are fine, because you are expecting the receipt from the doctor, and you are expecting the freebie, in fact it’s probably why you signed up for the mailing list.
And that’s our point. Don’t send attachments if the person on the other end isn’t expecting it. One reason for not sending attachments via email unless you first advise your correspondent to be on the lookout for the attachment is, of course, because the existence of the attachment (by which we mean a file included in the email) is likely to be scanned by the spam filtering and virus detection systems on the other end, which means that it may end up in the spam folder. If the person on the other end knows to be watching for your email with the attachment, they can rescue it from the spam folder.
Another reason is because even if your email does make it to the inbox, unless the receiver is expecting an attachment from you they may not open it (in fact, arguably they shouldn’t), for fear that your computer has been compromised and that instead of you sending them a file, it’s a virus that is sending itself out in your name.
Sending out a mass mailing with files attached can cause your mail to run into other problems, as well, starting even before it ever gets to the other end. Your own ISP may see the large stream of email with attachments coming from you and assume that you are sending out a virus – intentionally or otherwise – and block your mail from leaving their system. This is considerably more likely with a file that contains code as compared to a PDF containing text (which can often be scanned by the receiving system’s spam filter).
The best way to ensure that your email gets to the person to whom you are sending it when it carries an attached file is to make sure that the person on the receiving end is expecting it. Then let them know that you sent it, and ask them to confirm receipt.
If you are conducting a large email campaign, attachements eat up a lot of your system resources. In addition to the excellent reasons already stated, tecipient email clients also do not like to process large attachments. It is better to link to a file for download to save system resources for both the sender and the recipient.