Why does a one-page PDF from Canva weigh 8MB while a 50-page Word export is only 600KB? Why does your scanned payslip compress from 4MB to 300KB but a digital bank statement barely changes? The answer is DPI — dots per inch. Understanding how DPI and image compression interact in PDFs makes it much easier to choose the right compression settings and predict the result before you hit submit.
DPI stands for dots per inch — the number of pixels used to represent one inch of an image. Higher DPI means more pixels, more detail, and a larger file size. Lower DPI means fewer pixels, less detail, and a smaller file.
PDFs are a mixed format: they can contain vector elements (text, lines, shapes drawn with mathematical coordinates) and raster elements (photos, scans, embedded images stored as pixel grids). DPI only applies to the raster elements — the images. This distinction explains why the same compression setting produces dramatically different results on different PDFs.
When a scanner captures a physical document at 300 DPI, it stores 300 × 300 = 90,000 pixels per square inch. When that image is embedded in a PDF and later compressed, the tool reduces the pixel density to, say, 96 DPI — storing only 9,216 pixels per square inch. The image is smaller in bytes, looks nearly identical on a screen, and takes a fraction of the storage space.
In casual usage, DPI (dots per inch) and PPI (pixels per inch) are used interchangeably for digital images. Technically, DPI refers to physical printer output (how many ink dots per inch) while PPI refers to pixel density in digital files. For PDF compression purposes, the distinction doesn't matter — both terms describe the same concept: how many data points represent one inch of an image. This guide uses DPI throughout as it's the more commonly recognised term in consumer contexts.
The relationship between DPI and file size is roughly quadratic: double the DPI and the image takes up approximately four times the storage space. Here's what that looks like in practice for a single A4 page:
| DPI | Pixels in an A4 image | Approx. file size (JPEG) | Best used for |
|---|---|---|---|
| 72 DPI | 595 × 842 px | ~50–150KB | Screen viewing, portal uploads, email |
| 96 DPI | 794 × 1123 px | ~100–300KB | Screen + light A4 printing |
| 150 DPI | 1240 × 1754 px | ~300KB–1MB | Good print quality, standard office printing |
| 300 DPI | 2480 × 3508 px | ~1–4MB | Professional print, press-ready files |
| 600 DPI | 4960 × 7016 px | ~4–16MB | High-resolution archiving, technical drawings |
A single A4 page with one embedded photo at 300 DPI occupies 1–4MB. A 10-page report with a photo on every page can easily reach 20–40MB before compression. After downsampling those images to 96 DPI, the same document might be 1–3MB — a 90%+ reduction with no visible difference when read on screen.
Canva and other design tools export PDFs with embedded images at 300 DPI or higher by default — optimised for commercial printing quality. A simple one-page design with background gradients, icons, and a photo can be 8–15MB because every graphical element is stored at full print resolution. Compressing to 96 DPI (Balanced mode) typically reduces a Canva PDF by 80–90% — a 10MB file becomes 1–2MB — with no visible quality difference when shared digitally.
This is the most important concept to understand — and the one that surprises most people.
Text in a PDF is not stored as pixels. It's stored as font instructions — mathematical descriptions of how to draw each character, along with references to font files embedded in the document. When a PDF viewer renders text, it draws it fresh at whatever resolution your screen requires. Text in a PDF is, in this sense, resolution-independent.
This means:
Open the PDF and try to select a word with your cursor. If the text highlights blue when you click and drag, it's real vector text — compression won't change how it looks. If you can't select any text, the document is image-based (a scan or screenshot) — every "character" is actually a pixel, stored as an image. Image-based PDFs compress dramatically better than text-based ones.
When you compress a PDF, the tool performs two main operations: downsampling (reducing image DPI) and recompression (applying more efficient compression algorithms to the pixel data that remains). Here's exactly what ShrinkPDF's levels do:
Applies minimal downsampling — images are reduced to around 150–200 DPI, and JPEG quality is kept high. File size reduction is typically 20–35%. Visually indistinguishable from the original even when zoomed in closely. Best for: portfolios, product photography, medical imaging, design work, or any situation where image fidelity is the priority and you're not constrained by a strict size limit.
Targets approximately 96 DPI with moderate JPEG quality settings. This is the sweet spot for most documents shared digitally. A 96 DPI image looks sharp on any standard monitor at 100% zoom — screens typically display at 72–110 PPI, so 96 DPI provides slightly more data than the screen shows, resulting in crisp rendering. File size reduction is typically 40–60%. Recommended for: email attachments, presentations, reports, mixed-content documents.
Targets approximately 72 DPI with aggressive JPEG compression. This matches the baseline screen resolution, so images look good at 100% zoom but become noticeably softer when zoomed in significantly. Produces the smallest possible output file — typically 65–85% smaller than the original. Recommended for: scanned documents, government portal uploads, any file that needs to meet a strict size limit (1–2MB). Text quality is completely unaffected.
Many people avoid Maximum compression because they fear their document will become "unreadable." In practice, for the documents most commonly compressed — payslips, transcripts, IC copies, bank statements, official letters — Maximum compression produces a file that is visually indistinguishable from the original when read normally on screen. The quality difference only becomes apparent when zooming in to 200%+, which is not how these documents are reviewed. A government portal reviewer reading your submitted payslip does not zoom in to inspect pixel-level image quality.
The DPI you need depends entirely on the final use of your document. Most people compressing PDFs are preparing files for digital sharing — email, portal upload, WhatsApp — not professional printing. This changes the requirement significantly:
For 95% of document sharing and portal uploads in Malaysia and Southeast Asia — where documents are reviewed on screen, not printed commercially — 72–96 DPI is completely adequate. You're not submitting your payslip to be printed on a billboard.
Not all PDFs compress equally, and understanding why helps you set realistic expectations:
| PDF Type | Main content | Typical compression reduction | Recommended level |
|---|---|---|---|
| Scanned document (colour, 300 DPI) | Image of page | 70–85% | Maximum |
| Scanned document (grayscale, 300 DPI) | Image of page | 60–75% | Maximum |
| Canva / design export | High-res images + vector | 75–90% | Balanced |
| PowerPoint / Slides export | Mixed text + images | 50–75% | Balanced |
| Word / Google Docs export | Text + occasional images | 30–60% | Maximum or Balanced |
| Text-only PDF (no images) | Pure vector text | 5–20% | Either (minimal impact) |
| Already compressed PDF | Mixed | 5–25% | Balanced |
The key pattern: documents with high-DPI images (especially scans) compress dramatically. Documents that are already digital text compress minimally — not because the tool is ineffective, but because there's very little pixel data to reduce.
If you're scanning physical documents for portal uploads or email, choosing the right scan settings eliminates the need for post-processing compression entirely. These settings produce portal-ready files directly:
A single A4 page scanned at 150 DPI in grayscale with Microsoft Lens typically produces a PDF of 50–200KB per page — a 10-page document comes in under 2MB with no compression needed. Compare that to the same document scanned at 300 DPI in colour (3–5MB per page, 30–50MB total) and the difference in approach is stark.
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