Converting DOC or DOCX to PDF fixes the document's appearance so it looks the same for everyone who opens it — fonts, margins, page breaks and all. That is the whole reason the format exists: a Word file reflows according to the fonts, printer drivers and application version on the reader's machine, which is why a document that was two pages when you sent it arrives as three. Conversion runs on our server using a real document engine, since rendering Word's layout faithfully is not something a browser can do: the file is streamed through and the PDF streamed back, with nothing stored — but the file does leave your device. Fonts are embedded into the result, so the document renders correctly on machines that do not have them installed, which is the other half of what makes a PDF portable.
Why a Word file looks different elsewhere
A DOCX describes content and formatting rules, then leaves the layout to be computed when it is opened. That computation depends on the fonts installed, the default printer's page metrics and the version of Word doing the work — so the same file legitimately produces different pagination on two machines.
PDF stores the finished layout instead. Every glyph has a fixed position and the fonts travel with the file, so what you approve is what the recipient sees. For anything being signed, filed, printed or submitted, that difference is not cosmetic.
What converts faithfully, and what to check
Standard documents — text, headings, tables, images, headers and footers — convert accurately, and internal structure like a table of contents generally survives as working links.
Worth checking: documents relying on unusual fonts, where a substitution shifts line breaks; content in text boxes and floating frames, which can move; and tracked changes and comments, which are either rendered or dropped depending on how the document was set up. Accept or reject changes before converting rather than discovering them in the output.
Before you convert
Finish the document first. A PDF is a poor editing format, so the natural order is to complete the Word file, convert, and keep the DOCX as the source you revise next time.
If the document has to be filled in or signed by someone else, decide which format they need. A PDF with form fields or a signature block works well; a PDF of a form expecting handwriting means printing and scanning, which is worth avoiding if the recipient has any alternative.