The 20 MB applies to the whole message
Apple states it as a per-message ceiling that includes the attachment, so the usable attachment size is lower than 20 MB. Encoding takes about a third on top of the raw file, which puts the practical limit near 14-15 MB for a PDF.
The iCloud Mail help also warns about a second layer: if you use a mail app rather than the web interface, messages under 20 MB can still fail to deliver, because your internet provider may enforce its own ceiling. When a 12 MB attachment bounces from a desktop client but sends from iCloud.com, the provider is the limit, not Apple.
There is a third ceiling people trip over without noticing: iCloud Mail is tied to your iCloud storage. If the account is full, mail stops arriving and sending fails, regardless of attachment size.
What Mail Drop actually does
Mail Drop uploads the file to iCloud and puts a link in the message. Apple's limits page says an attachment can be up to 5 GB this way, that Mail Drop attachments do not count against your iCloud storage, and that the recipient has 30 days to download.
Thirty days is the part that bites. A Mail Drop link in a message someone reads three weeks later still works; the same message forwarded on in month two does not. And because it is a link, it fails in exactly the places an attachment would have worked: application portals, HR inboxes that strip links, and any recipient who needs the file saved alongside the email.
The storage quota is shared too - Mail Drop has its own 1 TB allowance, separate from your iCloud plan, and expired attachments free it up again.
Compress, or send a link?
Compress when the document has to live inside the message: a signed contract a client will forward, a scan an office will file, anything a portal will ask for later. A file under 14 MB arrives as a file, and keeps working when the message is forwarded.
Use Mail Drop when the file is genuinely large and the recipient is a person, not a system - a photo set, a video, a design export. The original bytes arrive intact, which compression cannot promise.
Making a PDF fit iCloud's ceiling
Aim at 14 MB rather than 19 MB so encoding and provider limits both have room. If the PDF is mostly text, rebuilding it usually gets there without touching a single pixel, and the text stays selectable, searchable and copyable.
If it is a scan, the pages are already pictures, so the file shrinks by re-encoding them at a lower resolution - which is a real quality trade, not a free win. Check the result at 100 percent before you send a document that has to be read closely.