Why PDFs get big
Most size comes from images, especially scans at high resolution. Text alone is small.
OmniDoc is still under development; this guide describes general methods that work with common Android PDF tools.
How compression works
Compression reduces image resolution and applies stronger image encoding. It can also remove unused data.
Choosing settings
For documents to read on screen, around 150 dpi is often enough. For printing, keep higher resolution. Try a medium setting first and check readability.
Check the result
Zoom in on small text and signatures after compressing. If unreadable, use a lighter setting.
Understanding resolution
Resolution, measured in dots per inch, controls how much detail an image holds. A page scanned at 300 dpi holds four times as many pixels as the same page at 150 dpi, so it is roughly four times larger before compression. For reading on a phone or laptop, 150 dpi is often sharp enough, while documents that will be printed or carefully inspected may need more. Choosing resolution deliberately is the single biggest lever on file size.
Image quality settings
Many compressors offer low, medium and high quality presets. Lower quality applies stronger JPEG-style compression, which can create blocky artefacts around text and lines. For text-heavy scans, a medium setting usually keeps text crisp while reducing size a lot. Photos tolerate stronger compression than diagrams and signatures.
Colour versus greyscale
Converting colour scans to greyscale can cut size significantly when colour adds nothing, such as for printed forms and letters. Keep colour where it carries meaning, like charts, stamps or photos. Some tools let you convert individual pages, which is useful for mixed documents.
A practical workflow
Make a copy of the original. Try a medium compression preset. Open the result and zoom into the smallest text and any signatures. If everything is readable and the size meets your limit, you are done. If not, try a stronger preset, or reduce resolution only on large image pages. Always send the compressed copy and keep the original archived.
Upload limits
University portals, government websites and email providers often set file size limits. Check the limit before compressing so you know your target. If a file still exceeds the limit after reasonable compression, splitting it into parts may be acceptable; read the portal's instructions.
Why some PDFs barely shrink
If a PDF is mostly text and vector graphics, it may already be small and compression will change little. Files that were exported from office software at sensible settings also tend to be efficient. Big gains come from scanned pages and large photos. If compression does not help, check whether the file contains embedded fonts or attachments that can be removed.
Avoiding repeated compression
Compressing an already compressed file again usually degrades images further with little size benefit. Always start from the original when you need a different size.
Compressing before you scan
The easiest way to get a small PDF is to scan sensibly from the start. Use your scanning app's document mode, choose a moderate resolution, crop to the page, and use greyscale for plain text documents. A file created this way often needs no further compression at all.
Naming compressed copies
Add a suffix such as -small to the compressed file name so you never confuse it with the original.
Alternatives
Scanning in greyscale or splitting a file can also meet upload limits.
Frequently asked questions
Does compression delete text?
No, it mainly reduces images.
Can I undo compression?
No; keep the original.
What size do portals accept?
It varies; check each portal’s limit.
What dpi is best for email?
Around 150 dpi is usually enough for on-screen reading.
Does compression affect signatures?
Strong compression can blur them; check after compressing.