Reducing PDF File Size: What Works and What It Costs
A PDF that is too large for an email or an upload form usually has one cause: images, most often scanned pages. This guide explains what takes up space in a PDF, how compressors shrink it and what they give up in return, how to keep files small from the start, and what to do when a size limit is still too tight.
What Makes a PDF Large
Text costs very little. A page of typed text with its fonts embedded is often only a few tens of kilobytes, while a single photograph or scanned page can take several megabytes. An A4 page scanned at 300 DPI in colour is 2,480 × 3,508 pixels, about 25 MB before compression; even a well-compressed JPEG of it is often around a megabyte.
| Content | Effect on size |
|---|---|
| Scanned pages | Usually the largest part; colour scans at high resolution most of all |
| Photos and screenshots | Large when they are placed at camera resolution and only shown small |
| Embedded fonts | Tens to hundreds of kilobytes each; full East Asian fonts can reach several megabytes |
| Merged files | The same logo or font can be stored once per source file |
| Edit history | Some editors append changes to the end of the file instead of rewriting it |
| Attachments | Files embedded inside the PDF count in full |
Before compressing, look at what the file contains. If the text can be selected and the pages hold few images, the file is probably compact already and a compressor has little to gain. If the pages are scans or full of photos, that is where the size is.
Two Ways to Compress a PDF
PDF compressors work in one of two ways. Some recompress only the images embedded in the file, at a lower resolution or quality, and leave text and vector graphics untouched. Others render every page as an image and build a new PDF from those images.
The LeedyBox Compress PDF tool uses the second approach: each page is drawn in your browser, saved as a JPEG and placed into a new PDF with the same page size. This shrinks scanned documents and photo-heavy files well, and the size of the result depends on the resolution and quality you choose rather than on how the original was made. The price is that the text becomes part of the image.
What the LeedyBox Compression Levels Do
Each level draws pages at a fixed resolution and saves them at a fixed JPEG quality. The pixel sizes below are for one A4 page (210 × 297 mm); a US Letter page is slightly wider and shorter.
| Level | Resolution | JPEG quality | A4 page in pixels |
|---|---|---|---|
| Recommended | About 86 DPI | 60% | 714 × 1,010 |
| Less Compression | About 108 DPI | 75% | 892 × 1,262 |
| Extreme Compression | About 65 DPI | 40% | 535 × 757 |
These resolutions suit reading on a screen. Body text stays readable at Recommended, footnotes and fine print are clearer at Less Compression, and Extreme Compression is for meeting a strict limit when some blur is acceptable. None of the levels is meant for printing, which usually needs around 300 DPI; keep the original file for print and for your archive.
What Is Lost When Pages Become Images
- Text. It can no longer be selected, searched or copied, and screen readers cannot read it.
- Links and bookmarks. Links stop working and the bookmark panel is empty.
- Form fields and comments. They stop working; what they showed can remain visible as part of the image.
- Digital signatures. The new file has no signature, even if the picture of one is still visible, so a signed contract should be sent as the signed original.
- Document details. The title, author and other document properties of the original are not carried over.
If any of these matter, the page-to-image approach is the wrong tool for that file. Use one of the alternatives below, or a desktop program that recompresses only the images.
Smaller From the Start: Scanning and Exporting
The settings used to create a PDF decide most of its size, so it is often easier to make a smaller file than to shrink a large one.
- Scan text in black and white or greyscale. A 300 DPI black-and-white page is about 1 MB before compression and usually much less after, compared with about 25 MB for the same page in colour. Use colour only for photos, stamps or coloured markings.
- Choose 200 to 300 DPI for documents. Higher settings mostly add size; keep them for small print or for archives that may be printed.
- Export with a size option. Word's "Save as PDF" offers "Minimum size (publishing online)", and many office and design programs have a similar setting that lowers image resolution.
- Resize photos before placing them. A photo shown 8 cm wide does not need the 4,000 pixels a phone camera produces.
- Save as a new file. "Save As" rewrites the PDF and can drop an edit history that "Save" would keep appending to.
When Compression Is Not Enough
Upload forms and email services set their own limits; Gmail, for example, accepts attachments up to 25 MB, and many application portals allow far less. If the compressed file is still too large:
- Split the document into parts that each fit, with Split PDF.
- Remove pages the recipient does not need, such as blank pages or duplicate scans, with Organize PDF.
- Rescan the worst pages in greyscale at a lower resolution instead of compressing them twice.
- Share a link from a cloud storage service when the recipient accepts one.
Compressing an already compressed file again usually saves little and blurs the pages further, so start each attempt from the original.
Frequently Asked Questions
Why did my scanned PDF barely get smaller?
Many scanners already compress their output, often as black-and-white images with very efficient methods such as CCITT or JBIG2. Turning those pages into JPEGs can save little or even add size. Compare the result with the original and keep the smaller file.
Can I print a compressed PDF?
You can, but it will look soft: printing usually needs around 300 DPI, and the levels in the LeedyBox tool draw pages at 65 to 108 DPI. Print from the original file.
Is it safe to compress a signed contract?
The compressed file no longer contains the digital signature, only its picture. For documents whose signature must be verifiable, send the original or ask the recipient for another way to deliver it.
Does the PDF leave my computer when I compress it?
No. The LeedyBox tool reads and rebuilds the file in your browser and saves the result as a new download named after the original, ending in _compressed.pdf.