How-to

How to reduce a PDF or photo file size for upload without losing readability

Why files get large

File size usually comes down to a few things: how detailed the image is, whether it is in colour, and how many pages there are. Common causes include:

• Phone photos taken at the highest resolution, which can be much more detailed than a page of text needs.
• Full-colour scans of pages that are plain black text on white paper.
• Scanning at a very high resolution setting.
• Many pages combined into one file.

Knowing which of these applies makes it easier to choose the right fix, rather than shrinking everything as far as possible.

Check the stated limits first

Before changing anything, it can help to look at what the upload page itself says about file size and file types. The upload page or current guidance on GOV.UK states the limits, and they can differ between services and change over time, so it is worth reading them rather than relying on a figure remembered from elsewhere.

Knowing the limit also avoids over-compressing. A file that is already within the stated limit may not need any reduction at all.

General ways to make a file smaller

Some people find these approaches useful, roughly in this order:

1. Scan at a moderate resolution. For pages of text, a setting somewhere around 150 to 300 dpi is a common middle ground between sharp text and a manageable file.
2. Use greyscale for plain text pages. A black and white or greyscale scan is often far smaller than a colour one. Colour may still be useful where a page has coloured stamps or logos.
3. Use a reduce-size or optimise option. Many PDF tools and phone scanner apps offer a compress, reduce size or lower quality export setting when saving.
4. Resize photos. A photo can often be exported at a smaller pixel size before being placed in a PDF.
5. Split a large file into logical parts. One approach is to divide a big file into sensible sections, such as one file per document type, if the upload page allows several files.

A separate guide on naming and numbering scanned files describes one way to keep split files easy to tell apart.

Check readability after compressing

Smaller files are only useful if the content is still clear. After compressing, it can help to open the new file and check it properly:

• Zoom in to a normal reading size and read a few lines of text.
• Look at small print, such as footnotes or reference numbers.
• Check that stamps, signatures and dates are still legible.
• Check figures, for example amounts and account numbers, character by character.
• Flick through every page in case one has come out blurred or cropped.

If something is hard to read, one option is to try a milder compression setting and compare the results.

Keep the original as well

Compression discards detail, and it cannot be reversed. Many people keep the original full-quality scan or photo in one place and save the reduced copy separately, with a clear file name showing which is which. That way a fresh copy can be made later if a different size or quality is needed. If the reduced files are later combined into a single document, the indexed PDF bundle guide describes how a combined file and its index can fit together.

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