As shared, "I have a problem with trying to email pdfs to one of my colleagues". Would you mind confirming how exactly you have been sharing the pdfs? Were you emailing it directly or using Adobe Acrobat/Reader's attach to email option for sharing?
For first scenario, if sharing pdfs through email then there's isn't anything that we can do about it. Its something related to either your or your colleague's email account's settings. May be his/her server is blocking the email attachments.
For the second one, if you can see the attachments in sent items and other people are able to receive emails along with pdf attachments, then again your colleague needs to check the email account setting. Probably an update, or change/reset of web browser would help.