iOS tells you the attachment can't be previewed. The file downloaded fine — iOS simply has no reader for it.
🔒 This file never leaves your device — nothing is sent to a server
The iOS preview engine supports a fixed list of formats: PDF, Office documents, images. HWP is not on it, and never will be — the format is used almost entirely inside South Korea.
Most HWP viewer apps on the App Store upload your document to their own servers to convert it. That is a poor trade for a transcript, a contract or a government form. This page does the work on your phone, so the file never leaves it.
Save the file first
From Mail, long-press the attachment and choose Save to Files. From KakaoTalk, tap the file then Share → Save to Files.
Open this page in Safari
Then tap the box above.
Choose 'Browse' and pick the file
It opens right there in the browser.
Keep it as a PDF
Tap Save as PDF, then Save to Files from the share sheet. The Files app can open it directly afterwards.
Yes, identically. On iPad you can also drag the file from the Files app into Safari in Split View.
About 1 MB once, when the page loads. The document itself is never transmitted, so its size doesn't matter.