Guides
Reduce PDF file size: guides built on measurements
Each guide answers one situation and backs it with numbers we measured ourselves, on public PDFs anyone can download, using the same code that runs on the PDFnook tool pages.
- Why is my PDF so large? Four real files, four different causesWe took four public PDFs apart byte by byte. Photos, one embedded font, accessibility tags and old saved versions each made a different file large.
- How to reduce PDF file size without losing qualityLossless compression changed four real PDFs by −11% to +9%. What “without losing quality” can mean in practice, with sizes and side-by-side crops.
- How to reduce PDF file size on a Mac (and why Preview can make it bigger)We ran macOS 26.2's Reduce File Size filter on four public PDFs: two got bigger, one shrank. What it does, when it helps, what worked better.
- PDF too large to email? How small it needs to be, and how to get thereGmail allows 25 MB, Outlook and iCloud 20 MB, and attachments grow about 37% when sent. How small the PDF really needs to be, tested on a 72 MB file.
- How to reduce the size of a scanned PDF and keep it readableA scanned PDF is a picture of every page. Tested on a typed speech draft: which settings cut 21% with searchable text kept, and which cut 91%.
- Photos to PDF with a small file size: choose the page size firstTwelve camera photos made a 72 MB PDF. With A4 pages, shrinking photos gave 8.7 MB at 200 DPI; with fit-to-image pages, the same setting barely helped.
How we tested
All guides use the same files and the same method, described once here.
The files
| File | What it is | Size |
|---|---|---|
| IRS Publication 17 (2025), Your Federal Income Tax | Text-only, 142 pages, tagged for accessibility | 2.95 MB |
| Kennecott Mines brochure, Wrangell-St. Elias National Park and Preserve | Designed brochure: vector text over large photos, 2 A2 pages | 690 KB |
| Wrangell-St. Elias National Park and Preserve official brochure | Each A2 page is one large JPEG with a hidden OCR text layer | 5.72 MB |
| Gerald R. Ford, “Mr. Ford of Michigan” speech draft (Ford Congressional Papers, Box D13) | Typewritten page scanned in greyscale, with an OCR text layer | 707 KB |
| Twelve NASA astronaut photographs (iss071e014284 and eleven others) | Nikon Z 9 and D4 camera JPEGs, 8,256 × 5,504 and 4,928 × 2,768 pixels | 71.8 MB |
Downloaded on 29 October 2026. U.S. government works are not protected by copyright (17 U.S.C. § 105); NASA describes its imagery as generally not subject to copyright. Third-party photos inside the brochures are not reproduced here.
What we ran
- PDFnook
- The tool pages' own modules, loaded in Chrome/156.0.8078.73 on a Mac (Apple M2, macOS 26.2): the lossless pass (Keep text), Shrink photos, keep text and Image-based copy at the resolution and quality choices offered on the Compress page (High 0.8, Medium 0.65, Low 0.45, Very low 0.32). Screenshots show the real pages running on these files.
- macOS
- PDFKit, Apple's PDF framework, writing each file with no options, with its “Save images as JPEG” and “Optimize images for screen” options, and with the system “Reduce File Size” Quartz filter.
- Byte breakdown
- A short pypdf script that sizes every stream in a file as stored and groups it as images, fonts, page drawing instructions, other drawings, metadata or structure. The same script measures each file inside a ZIP archive (Python's zipfile, Deflate level 9).
What we measured
- Size of every output file, in binary units as the tool shows them: 1 KB = 1,024 bytes, 1 MB = 1,048,576 bytes. Repeated runs agreed to within a few dozen bytes.
- Selectable text: the characters (spaces not counted) that pdf.js, the PDF renderer used by Firefox and by PDFnook, extracts from every page.
- Appearance: crops of the same page area rendered by pdf.js at 200 DPI (300 DPI for the tax guide and the photos) and placed side by side. Judging sharpness is left to your eyes; we do not quote a quality score.
What this does not show
Four PDFs and one set of photos are not every PDF. Results for another file can differ in either direction, which is why the tool reports the size before you download. We did not test Safari, Windows, iPhone or Adobe Acrobat.