A 24 MB PDF that refuses to upload is usually not a PDF problem. It is a settings problem. If you have ever compressed a file only to end up with blurry text, muddy images, or no meaningful size reduction at all, this guide to PDF compression settings will help you choose the right option first time.
Most people only want one thing: a smaller file that still looks right. The catch is that PDF compression is never just one switch. Size depends on what is inside the file, where it is going, and how much quality you can afford to lose. A scanned contract, a photo-heavy brochure, and a text-only invoice all need different treatment.
What PDF compression settings actually change
When you compress a PDF, the software usually adjusts three things: images, fonts, and hidden document data. Images have the biggest effect by far. If your PDF contains full-page scans or embedded photos, image settings will decide whether the file drops from 20 MB to 3 MB, or barely changes.
Fonts are another factor. Some PDFs embed entire font families even when only a few characters are used. Good compression can subset fonts so the file only keeps what it needs. Metadata, annotations, form history, and unnecessary objects can also add extra weight, though usually less than images.
This is why two PDFs with the same page count can behave very differently. A ten-page scanned document may be larger than a hundred-page report made mostly of text.
A practical guide to PDF compression settings by file type
The fastest way to choose the right settings is to start with the type of PDF you have.
Text-only PDFs
If your file is mostly digital text with a few simple graphics, use light compression. These documents are often already efficient, so aggressive settings bring little benefit. You may save a bit more by removing embedded thumbnails, unused objects, or excess metadata, but there is rarely a need to reduce image quality because there may be few images to reduce.
For text-heavy PDFs, the goal is to preserve sharp lettering. If the text was created digitally, it should stay crisp even at a low file size. If compression makes text fuzzy, the file may actually contain scanned pages rather than live text.
Scanned PDFs
Scanned files are the main reason people look for PDF compression. Each page is usually an image, so file size climbs quickly. Here, resolution and image quality matter most. If the scan is monochrome, such as a typed letter or a signed form, stronger compression often works well. If it includes colour diagrams, stamps, or photographs, you need a gentler setting.
For routine sharing and uploads, lowering scan resolution to around 150 dpi is often enough. For archive copies or pages with fine print, 200 dpi may be safer. Going much lower can make small text hard to read, especially on printed pages.
Image-heavy PDFs
Brochures, portfolios, menus, catalogues, and presentations usually contain photos. These need a more careful balance. Over-compress them and faces, product shots, and gradients start to break apart. Under-compress them and the file stays too large for email or web upload.
If the PDF is mainly for screen viewing, medium image compression usually gives the best result. If it is for print, keep higher image quality and accept a larger file. There is no setting that gives print-ready quality at tiny file sizes.
The settings that matter most
Different tools label options in different ways, but most PDF compressors are changing the same core controls.
Image resolution
Resolution is measured in dpi. Higher dpi keeps more detail but creates larger files. For on-screen reading, 100 to 150 dpi is often enough. For mixed documents that may be printed occasionally, 150 to 200 dpi is a safer middle ground. For professional printing, you may need 300 dpi, but that is rarely the right choice if your main goal is a much smaller upload file.
A common mistake is compressing a scan from 300 dpi to 72 dpi. Yes, the file gets smaller, but text on scanned pages can become rough and tiring to read. For forms, invoices, and classroom handouts, 150 dpi is usually a more useful target.
Image quality and compression method
Many tools offer low, medium, and high compression, or a quality percentage. These settings affect how much image data is discarded. JPEG-style compression is common for photographic content, while monochrome scans may use a different method that keeps black text fairly clean at small sizes.
If your PDF contains photos, start in the middle rather than at the lowest quality. Extreme compression often creates visible artefacts around edges and blocks of colour. If your PDF is a simple black-and-white scan, stronger compression may still look acceptable.
Colour conversion
Some compressors reduce file size by converting colour images to greyscale. This can help a lot when colour is not essential. For contracts, lecture notes, and administrative documents, greyscale is often fine. For brand assets, reports with colour-coded charts, or marketing PDFs, it can create confusion or make the file look poor.
There is also monochrome conversion, which makes files even smaller. Use it only for plain text scans. On anything with shaded graphics or photos, it usually looks too harsh.
Font embedding
If your PDF contains live text, font handling matters. Subsetting fonts can reduce file size without changing appearance. Removing embedded fonts entirely can save more space, but it risks display issues if the recipient’s device does not have the same fonts installed. For general sharing, keeping essential font data is the safer choice.
Object and metadata cleanup
Some PDFs carry extra baggage: unused objects, embedded previews, comments, hidden layers, and metadata. Cleaning these can trim size without touching visual quality. The savings are often modest, but this is the least risky place to start if you want a smaller file and cannot afford any visible drop in quality.
How to choose the right compression level
If your tool offers broad presets such as low, medium, and high compression, think in terms of purpose rather than the label.
Low compression is best when appearance matters more than size, such as client documents, design proofs, or print files. Medium compression suits most everyday use – sharing by email, uploading to portals, or storing documents neatly without making them look poor. High compression is for strict size limits, especially with scans, but it should always be checked page by page afterwards.
This is where a free, in-browser tool can save time. Instead of installing software or creating an account, you can test one version at medium compression, compare the result, and only go stronger if needed. That simple approach is often faster than guessing.
Common mistakes when changing PDF compression settings
The biggest mistake is compressing the same file repeatedly. Each pass can reduce quality further, especially when images are reprocessed. Always go back to the original PDF if you need to try different settings.
Another mistake is using one preset for every file. A scanned ID document and a product brochure should not be treated the same way. Compression is not about finding the best universal setting. It is about matching the setting to the job.
Many users also focus only on file size and forget readability. Saving 2 MB is not a win if signatures become faint or small print becomes difficult to read. Always zoom in on key pages before you send or upload the result.
Recommended starting points
If you want a simple baseline, use 150 dpi with medium image compression for general documents. For scanned forms and admin paperwork, try greyscale if colour is unnecessary. For image-heavy PDFs, stay with colour and choose medium rather than high compression. For print-focused files, compress lightly and accept that the result may remain larger.
These are starting points, not rules. If your upload limit is very strict, you may need to push harder. If the document is going to a client, a solicitor, or a printer, keep quality ahead of file size.
When compression will not fix the real problem
Sometimes the PDF is large because the source file was poorly made. Oversized scans, duplicate pages, unnecessary high-resolution images, and exported design files with print settings can all create bloated PDFs. In those cases, compression helps, but rebuilding the PDF from a better source can work far better.
It also helps to ask what the file is for. If you only need to send a draft for review, a smaller compressed copy is sensible. If you need a master archive, keep an original version too. One file does not always need to do every job.
If you want the easiest rule to remember, it is this: compress for the destination. Screen reading, upload limits, and print each need a different balance. Pick the setting that fits the task, check the result properly, and your PDF will usually be small enough without looking cheap.