PDF File Size Guide: Why PDFs Are Large and How to Fix Them
PDF file sizes vary enormously — from a few kilobytes for a plain text contract to hundreds of megabytes for an engineering drawing with embedded fonts and high-resolution renders. Understanding why determines how to fix it.
Typical PDF sizes by document type
Here are realistic file size expectations:
- Simple text contract (5 pages): 50–200KB - Report with charts and some images (20 pages): 500KB–3MB - Scanned document (10 pages, 300 DPI): 3–8MB - Product brochure with high-res photos (12 pages): 10–50MB - Engineering drawing or CAD PDF: 20–200MB - eBook with images (300 pages): 50–200MB
If your PDF is larger than these benchmarks for its type, there is likely room for compression.
The 6 causes of large PDFs
1. **High-resolution embedded images** — A photo embedded at 300 DPI for print takes up far more space than the 72–96 DPI needed for screen viewing. Reducing DPI is the highest-impact compression technique.
2. **Scanned pages stored as images** — Each scanned page is a photograph. At 300 DPI, an A4 page is roughly 3–5MB before compression.
3. **Unoptimised fonts** — Fully embedding a font family adds 0.5–2MB. Subsetting (embedding only the characters used) reduces this to under 100KB.
4. **Revision history / undo data** — Some PDF creation tools store document revision layers. A file edited 10 times in Adobe Acrobat may retain all 10 versions internally.
5. **Attached files** — PDFs can contain embedded attachments. A 50MB Excel file attached to a 1-page PDF means a 51MB PDF.
6. **Non-optimised export settings** — Printing to PDF from Microsoft Office with 'Best for printing' rather than 'Best for electronic distribution' produces files 3–5× larger than necessary.
Compression methods and their effectiveness
| Method | Typical size reduction | Best for | |---|---|---| | Image DPI reduction (300→96) | 60–80% | Image-heavy PDFs | | Grayscale conversion | 20–40% | Colour documents where colour isn't needed | | Font subsetting | 5–15% | Fonts-heavy PDFs | | Remove revision history | 10–30% | PDFs edited multiple times | | Linearisation (web optimise) | 2–5% | PDFs served from web servers | | Re-export from source | 40–70% | PDFs created with 'print' export settings |
When compression is not enough
If your PDF contains genuinely large content (a 200-page illustrated textbook, a high-fidelity engineering drawing), aggressive compression will visibly degrade quality. In these cases:
- Split the document and distribute only the relevant pages - Share via a cloud link (Drive, Dropbox) instead of attachment - Use a print-on-demand service for physical distribution - Separate the document into a 'text-only' version for distribution and a 'full quality' master for archiving
Frequently asked questions
- What is a normal PDF size?
- A text-only PDF is typically 50–300KB. A report with images is 500KB–5MB. A scanned document is 1–10MB per 10 pages at 300 DPI. If your PDF is significantly larger than these ranges, it likely has unoptimised images or revision history.
- Why is my 1-page PDF 10MB?
- Almost always because it contains a high-resolution embedded image (a scan, photo, or diagram). At 300 DPI, a single A4 page image is 5–8MB before any compression. Compressing with WhiskFile's 'Small file' preset will typically reduce this to 200–400KB.
- Does the number of pages affect PDF file size a lot?
- Yes, but proportionally to content. A 100-page text-only PDF might be 500KB. The same 100 pages with one scanned image each could be 100MB. Page count matters less than image content per page.