PDF files can vary from a few kilobytes to hundreds of megabytes for documents of similar visual complexity. Understanding what drives PDF size lets you create smaller files from the start — saving storage, speeding up sharing, and avoiding email attachment limits.
Images are the single biggest contributor to PDF size. A raw 12MP camera photo is around 36MB uncompressed. Saved as JPEG at 85% quality, it's around 3–5MB. When embedded in a PDF, the compression used within the PDF container determines how much space the image occupies.
PDF supports several image compression formats: JPEG (lossy, best for photos), JPEG2000 (better quality-to-size ratio, supported in PDF 1.5+), ZIP/Deflate (lossless, best for graphics with flat colours), and CCITT/JBIG2 (monochrome, excellent for black-and-white scans). Many PDF creators use PNG or TIFF embedding (essentially lossless) by default, which keeps quality high but results in large files.
PDFs must embed font data to display correctly on any device. A complete OpenType font with all glyphs can be 500KB–3MB. PDFs that embed the full font file (rather than subsetting) bloat significantly. Subsetting — embedding only the characters actually used in the document — dramatically reduces font overhead. A document using 200 unique glyphs from a 5,000-glyph font needs only ~4% of the font data if subsetting is applied correctly.
When you edit and save a PDF without choosing "Save As" (or "Export as New"), most PDF editors append the changes to the end of the file rather than rewriting it. This preserves undo history but grows the file with every save. A document edited 10 times may contain 11 versions of some pages. "Flattening" a PDF by saving as a new copy collapses this history.
PDFs can carry extensive metadata: author, title, subject, creation date, modification history, software version strings, and extended XMP metadata packets. Well-intentioned metadata is useful; bloated metadata from verbose export tools adds unnecessary bytes.
PDFs can contain attached files — spreadsheets, images, other PDFs. If your document has attachments you don't need, removing them can dramatically reduce size. Use the PDF Info tool to inspect what's embedded in your document.
Obviously, more pages mean more data. But well-compressed pages add less per-page size than poorly-compressed ones. A 100-page text-only PDF with proper compression might be 500KB; a 100-page PDF of uncompressed scans could be 500MB.
Newer PDF versions (1.5, 1.6, 1.7) support cross-reference stream compression which can reduce the PDF structure overhead by 20–40% for large documents. Older PDF compatibility modes (targeting PDF 1.3–1.4 compatibility) cannot use these optimisations.
Adobe Acrobat Pro's PDF Optimiser has an Audit Space Usage button (File → Save As Other → Optimised PDF → Audit Space Usage) that shows a breakdown of space used by images, fonts, content streams, structure, and overhead. This tells you exactly where to focus your compression efforts.
For a free alternative, use Folium's PDF Info tool to see the file size and page count, then divide to get an average per-page size — this quickly reveals if a specific page is unusually large (suggesting an embedded high-resolution image on that page).
The best time to optimise a PDF's size is when creating it, not after. Recommendations by source application:
Microsoft Word / Google Docs: When exporting to PDF, look for quality or compression options. In Word for Windows: File → Export → Create PDF/XPS → Options. In Google Docs: there are limited size controls, but using File → Download → PDF maintains reasonable sizing for text documents.
Adobe InDesign: File → Export → Adobe PDF, then use the PDF/X-1a preset for press (larger) or the "Smallest File Size" preset for screen/web distribution. Adjust image downsampling settings (Compression panel) — 150 dpi for screen PDF, 300 dpi for print.
macOS Print to PDF: macOS Print to PDF tends to create relatively efficient PDFs from non-image documents. For image reduction, use the built-in Quartz filters in ColorSync Utility → Filters.
After the fact, the options depend on what's making the file large:
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