Short answer
If a PDF is too big to email, drop it here to re-optimize it in your browser and download a smaller version — no upload. Email limits are usually around 20–25 MB; this removes redundant internal data to help you slip under them.
Most email providers cap attachments near 20–25 MB. A quick lossless pass often trims a bloated export enough to send. If a PDF is huge because of high-resolution scanned images, expect smaller gains from a lossless pass — the data itself is large.
The whole thing runs on your device, so a confidential attachment is never uploaded just to shrink it.
Commonly 20–25 MB (Gmail 25 MB, Outlook ~20 MB). Compressing can help a bloated PDF fit.
No — it's compressed in your browser and never leaves your device.