I've spent way too much time searching for this exact thing across Japanese and English platforms, so here's the messy reality. Yen Press, the official English publisher, offers samples through retailers like Amazon Kindle, Google Play Books, and Apple Books—you can usually download the 'Look Inside' or preview, which covers the first chapter or so. Their own website sometimes has excerpts too, but it's inconsistent.
That said, the samples are often frustratingly short, maybe 30 pages if you're lucky, just enough to get a taste of the ridiculous premise but not enough to see if the later lore hooks you. If you want more substantial free reading, you're better off looking at fan-translated snippets that pop up on aggregate sites, but the quality swings wildly and it obviously doesn't support the author. I miss the days when some publishers put whole first volumes up as previews.
Honestly, for a series as long-running as DxD, the first sample barely scratches the surface of the power scaling and mythology that develops later, which is a shame because the early chapters are mostly... well, the infamous fan service bits. I ended up buying the first volume digitally because the sample left me curious about where the actual plot was going, beyond the initial premise.