PNG to PDF

How PNG transparency works inside a PDF

PNG supports an alpha channel — a per-pixel transparency value that allows images to have soft edges, semi-transparent areas, and fully transparent backgrounds. When this tool embeds a PNG into a PDF using pdf-lib, the alpha channel is preserved in the PDF's image object.

However, PDF pages have a white opaque background by default, and most PDF viewers composite the transparent PNG against that white background when rendering. The result is that transparent areas appear white in the PDF viewer. If you need a different background colour, fill the transparent areas in an image editor before uploading. The transparency data remains in the PDF object itself — if the PDF is later opened in an application that supports custom page backgrounds, the original transparency will interact with whatever background is set.

PNG versus JPG: when PNG is worth the larger file size

PNG uses lossless compression, meaning every pixel value is stored exactly. JPG uses lossy compression that discards information imperceptibly (at high quality) or visibly (at low quality). For document conversion, the choice matters in specific situations.

Choose PNG when your images contain: sharp text or diagrams where compression artefacts would be noticeable, transparency or sharp edges, screenshots or interface mockups, or technical illustrations with flat colours. Choose JPG when your images are photographs or contain gradients and photographic textures — JPG will be dramatically smaller with no visible difference.

A 300 DPI PNG scan of a text page can be 3–5× larger than the equivalent JPG at quality 90. If file size is a concern and your PNGs are photographs, convert them to JPG first using the Image Compressor tool before creating the PDF.

Tips for combining multiple PNG files cleanly

Before uploading, verify that all your PNG files have consistent dimensions if you want uniform page sizing. Mixing portrait and landscape PNGs in the same PDF will result in pages of different aspect ratios, each fitted individually to A4 — which is technically correct but can look inconsistent in a viewer.

If your PNGs are scans of physical documents, ensure they are deskewed (straightened) and cropped to remove scanner borders before conversion. Most document scanners include auto-crop software, or you can use an image editor. Skewed scans embedded in a PDF cannot be straightened after the fact without re-processing the images.

For archival use — storing historical documents, certificates, or artwork — PNG-in-PDF is the right choice over JPG-in-PDF. The lossless encoding means the PDF can be re-processed decades later without accumulated compression loss.