How PDF-to-image rendering works
PDF rendering is a two-step process: parse the PDF's vector instructions (text positions, shapes, embedded images) then rasterize them to pixels at a target DPI. At 72 DPI, an A4 page becomes 595 × 842 pixels — fine for on-screen preview. At 150 DPI it's 1240 × 1754 — suitable for web display. At 300 DPI it's 2480 × 3508 — the threshold for professional printing. Higher DPI produces sharper text, especially for small fonts, at the cost of larger file size.
Browser-based PDF rendering uses PDF.js (Mozilla's open-source library, the same engine Firefox uses internally). It handles text fonts, embedded images, and most vector graphics correctly. Complex PDFs with unusual fonts, encryption, or advanced transparency features may render differently than in Adobe Acrobat.
PNG vs. JPEG for the output image
Choose PNGwhen the PDF contains text, diagrams, or screenshots — lossless compression preserves sharp edges and readable text at any size. JPEG compression blurs edges and creates artifacts around high-contrast text (the dark letters on white background pattern is the worst case for JPEG's DCT algorithm).
Choose JPEG only for PDFs that are entirely photographic (scanned photos, image-heavy documents with no text). JPEG at quality 85+ reduces file size by 60–70% vs. PNG with negligible visible quality loss on photographs. For a mixed document (text + photos), PNG is the safer choice.
Why some PDFs render as blank images
Three common causes: (1) Password-protected PDFs — the renderer cannot access content without the password. Remove protection in Acrobat or use a PDF unlocking tool first. (2) PDFs with only scanned images— if the "text" in the PDF is actually a scanned image at very low contrast, it may appear blank when the contrast isn't boosted during rendering. (3) Corrupted PDF structure — a partially downloaded or damaged PDF may parse with empty pages. Try opening in Acrobat to verify the file is intact before converting.
What DPI to use when extracting PDF pages as images
DPI (dots per inch) controls the output resolution. Higher DPI = larger file, sharper image. Here's what to choose for each use case:
| Use case | Recommended DPI | Output size (A4 page) |
|---|---|---|
| Web display / website embed | 72–96 DPI | ~595×842 px — small file, fast to load |
| Email attachment thumbnail | 96–150 DPI | ~794×1123 px — clear on screen, compact |
| Presentation slide (PowerPoint/Keynote) | 150 DPI | ~1240×1754 px — crisp on most projectors |
| General on-screen use | 150 DPI | Good balance of quality and file size |
| Print-ready (standard quality) | 300 DPI | ~2480×3508 px — recommended for printing |
| High-quality archival / OCR input | 300–600 DPI | Maximum clarity for text recognition |
| Large-format print (poster) | 300+ DPI | Size depends on print dimensions; bigger = more pixels |