Two formats, one name
Hangul Word Processor is the standard word processor in South Korea, and its documents are everywhere in Korean government, legal and academic work. People outside that world usually meet an HWP file the same way: someone sends one, and nothing they own will open it.
There are two formats behind the name. HWPX, from 2010 onwards, is a zip of XML in the OWPML schema — the same shape as .docx, .xlsx and .odt, and readable with the tools a browser already has. The classic .hwpis a Microsoft compound-file container holding zlib-compressed records in Hancom's own layout, with optional encryption. That one needs a dedicated parser and is refused here by name rather than failing halfway with an unhelpful error.
The font problem, stated plainly
An HWP document is almost always Korean, and this is where the tool runs into a wall that no amount of parsing fixes.
Drawing text into a PDF needs a font containing those glyphs. The fourteen standard PDF fonts — the ones every reader has built in, and the reason a PDF made here is a few kilobytes rather than a few megabytes — cover Western European characters and contain no Hangul whatsoever. Korean text therefore converts to question marks.
Fixing that means embedding a Korean font, and a font with full Hangul coverage is several megabytes that every visitor to the site would download. That is a real trade, and the choice here is to be clear about the limit rather than ship the weight. For a genuinely Korean document, Hangul's own PDF export is the right tool.
What it is good for
An HWPX whose content is Latin script — an English report written in Hangul, a document of names, codes, numbers or tables — converts cleanly, and that is a real case for anyone who receives HWP files and does not own the software.
It is also a way to see what is inside a file you otherwise cannot open at all: paragraphs and tables come through, so the structure and any Latin-script content are recoverable.
Privacy
The document is unzipped and read entirely in your browser. Nothing is uploaded, which is the point when the file came from a government or legal process.