You convert a Word document, open the PDF, and something's slightly off — a line wraps differently, a heading sits lower, a table edge is tighter. This is normal, and understanding why makes it easy to avoid.
Fonts are the usual culprit
Word lays out text using fonts installed on your computer. When a document is rendered somewhere else, any font that isn't available gets substituted, and the replacement rarely has the exact same letter widths. Slightly wider letters push a word to the next line, which cascades down the page. Sticking to common fonts — Times New Roman, Georgia, Arial, Calibri — avoids most of this.
Page breaks that aren't really page breaks
If you moved content to a new page by pressing Enter many times, that "break" is really just blank lines. Any shift in spacing moves it. Use a real page break (Ctrl+Enter in Word) wherever a new page must start, and it will hold its place through conversion.
Images and margins
Oversized images and unusual margin settings are the other common sources of drift. A photo pasted at full resolution can render at a different scale than you expect; resizing it to roughly its on-page size first keeps things predictable.
When you need pixel-perfect fidelity
For everyday documents the differences are usually invisible. But if you have a precisely typeset thesis or a designed layout where every millimetre matters, Word's own Save as PDF will match Word exactly, because it's the same engine doing both. For everything else, a quick in-browser conversion is faster and the result is faithful.
Always skim the finished PDF page by page before sending. It takes seconds and catches the rare shift before your reader does.