Most people assume email attachments top out at "25MB," but the actual limit varies a bit by service.
Gmail officially allows attachments up to 25MB; anything bigger is automatically swapped for a Google Drive link. Daum (Kakao) Mail, a major Korean webmail service, also caps regular attachments at 25MB and automatically switches to a large-attachment link beyond that — Daum's own help page doesn't specify the exact size cap after that switch, but in practice it's known to handle several GB. Naver Mail, another major Korean webmail service, sets its regular attachment limit lower, at 10MB. Past that it also switches to a large-attachment link (up to 2GB per file), but those links typically expire after 30 days or 100 downloads — so if the recipient opens it late, they may not be able to retrieve the file at all.
For anything important — a contract, a resume, a portfolio PDF — it's safer to shrink the file so it fits inside the regular attachment limit outright. Large-attachment links can expire, and some recipients' mail policies block them entirely.
With CornButter, you can enter an exact target size for a PDF, and the same goes for photos. To fit Naver's 10MB limit, for example, open Resize Image to Target Size or Compress PDF and enter 10240KB (10MB) as your custom target.