OfficePDF

Your scanned PDF is 30 MB. Here is why, and how to get it under 10 MB without losing legibility

2026-09-19 · 3 min read

Email systems typically reject file attachments larger than 25 megabytes. When you scan a physical contract or a lengthy report, the resulting PDF often exceeds this limit. Understanding how scanners capture data explains why these files grow so large and how you can reduce them efficiently.

A scanner acts like a digital camera taking a close-up photograph of every single page. The final file size depends heavily on the resolution and color settings chosen before the scan begins.

The hidden cost of default scanner settings

Many office scanners default to a resolution of 300 or even 600 dots per inch (DPI) in full color. This setting is optimal for printing high-quality photographs, but it is entirely unnecessary for standard black-and-white text documents.

An A4 page measures 8.27 by 11.69 inches. At 300 DPI, the scanner captures over 8.7 million pixels per page. In full color, this raw data requires roughly 25 megabytes of memory per page before compression. Even after the scanner applies standard JPEG compression, a single page can still consume 1 to 2 megabytes. A standard 20-page contract scanned at these settings easily results in a 30 to 40 megabyte file.

Why 150 DPI is the standard for screen reading

Reducing the scanning resolution is the most effective way to manage file size. For documents that will be read on a computer screen, 150 DPI provides a clear, legible image without unnecessary bulk.

At 150 DPI, the scanner captures only one quarter of the pixels compared to 300 DPI. This reduction typically brings the file size down to between 200 and 400 kilobytes per A4 page. A 20-page document scanned at 150 DPI usually results in a manageable 4 to 8 megabyte file, which passes easily through any email gateway. The text remains sharp on standard monitors and is perfectly acceptable for archival purposes.

Grayscale versus color channels

Color mode is the second factor driving up file size. A full-color scan records separate values for red, green, and blue channels for every single pixel. This triples the amount of data collected compared to a simple black-and-white or grayscale image.

Switching the scanner or the compression setting to grayscale removes this extra color data. This adjustment alone generally saves another 20 to 40 percent in file size. Signatures in blue ink or colored corporate logos become gray, but the document retains its full legal and communicative value.

Processing existing large files

When you receive a massive PDF that someone else scanned, you need a way to reduce it after the fact. The solution is to process the document through software that re-renders the existing pages at a lower resolution.

Instead of dealing with overly large attachments, you can use Compress PDF to re-render each page as a JPEG at 100, 150, or 200 DPI directly in your browser. Selecting the grayscale option in Compress PDF provides further size reduction without ever uploading your confidential business files to an external server.

The trade-off with text-based documents

It is important to distinguish between scanned image PDFs and text-based PDFs. A document saved directly from a word processor contains text characters and embedded fonts, not a grid of pixels. These files are naturally very small, often under 1 megabyte even for hundreds of pages.

Applying image-based compression to a true text PDF does not reduce its size and has a side effect. The process flattens the document, converting crisp vector text into a flat image. This means the output text is not selectable or searchable. You lose the ability to search for keywords or copy paragraphs into another application.

Splitting as a final resort

Sometimes a document is simply too long. A 200-page court transcript or a massive technical manual scanned at even modest settings might still exceed email limits.

When resolution and color adjustments are not enough, or when you must preserve a text-based document's searchability, the only remaining option is to divide the file. Splitting the document into two or three smaller sections allows you to send the information across multiple emails without altering the underlying quality or structure of the pages. Before splitting, always check if Compress PDF can solve the issue for image-heavy documents, as keeping a single file is usually preferred by recipients.