About Word to PDF
Word documents look different on every machine. A font your office has on Windows isn't installed on the recipient's Mac. Track changes shows up as messy markup the moment somebody opens the file in Pages instead of Word. The signature line that lined up perfectly on your screen jumps half an inch on theirs.
Converting to PDF locks the file. Fonts get embedded, the layout freezes, and whoever opens it sees exactly what you saw. PDF is what you send when the formatting matters more than the ability to edit, which is most of the time when you're sending a finished document.
What goes in, what comes out
DOCX is the format Word uses now, and it converts cleanly. DOC, the older binary format from Office 2003 and earlier, also works. RTF, the cross-platform rich text format, converts fine. ODT, the OpenOffice and LibreOffice format, is supported too. TXT goes through, though for a text file you'll usually want a fixed-width page layout, which the converter applies automatically.
The output PDF preserves your fonts (embedded as subsets so the file isn't bloated), images at their original resolution, tables with their borders and shading, headers and footers, page numbers, and footnotes. Hyperlinks stay clickable in the PDF.
Things that translate well
Standard prose, bullet lists, numbered lists, headings, and styles. These are the bread and butter of Word and they come out looking the same on the other side.
Tables. Word tables convert cleanly including merged cells, shaded rows, and cell borders. Page breaks inside tables get respected.
Images and shapes. Inline images stay where you put them. Floating images keep their wrap settings most of the time, though anchored images at the edge of a page sometimes shift by a few points.
Track changes and comments. By default the converter accepts all changes and drops the comments, so the PDF shows the clean final version. If you want comments in the output, there's an option to include them as annotations.
Where it gets tricky
Fancy SmartArt graphics from Word are flattened to images. They look fine in the PDF but they're no longer editable. Same with WordArt and 3D effects, which were rare in modern documents anyway.
Linked content. If a Word doc has a linked Excel chart that updates from a spreadsheet, the PDF freezes whatever the chart looked like at conversion time. The link is broken in the PDF by design.
Very long documents over 500 pages can take a minute or two to convert. The progress bar shows where it is so you know it hasn't hung.
When you'd reach for it
Sending a contract for signature. PDF is the universal format for signed agreements, and most e-signature tools want PDF input. Word files get flagged by recipients who don't want to install Office just to read the document.
Submitting to a journal, university, or government portal. These almost always require PDF. Some explicitly reject Word files.
Sharing a finished report or proposal with a client. PDFs look professional, prevent accidental edits, and open the same on every device.
Archiving. Word formats change over the years. Old DOC files from 2005 can be a pain to open today. PDFs from the same era still open in any reader without complaint.
About the conversion
Converting Word to PDF in a browser is tougher than the other way around, because Word's layout engine is closed and proprietary. This tool uses LibreOffice on a server to do the heavy lifting, then ships the PDF back. So Word and PowerPoint and Excel conversions involve a server round trip, while pure browser tools like merge and split don't. Your file lives on the server only for the few seconds it takes to convert and is deleted as soon as the PDF goes back to you.