What is file compression?
File compression is the process of rewriting a file so it takes up less space. Lossless compression (ZIP, PNG) rebuilds the original exactly; lossy compression (JPEG, WebP, MP3, MP4) also removes detail you're unlikely to notice to get much smaller files. Compressed files are quicker to email, upload, store and download. For the mechanics behind it, see how file compression works and lossless vs lossy compression.
What File Compression Does
File compression reduces the number of bytes needed to represent the same data. There are two fundamental approaches:
Lossless compression — removes redundant data that can be perfectly reconstructed. Opening the compressed file gives you exactly the original. Examples: ZIP, PNG (when saved optimally), GIF.
Lossy compression — discards data that human perception does not notice strongly. The original cannot be reconstructed exactly, but the result looks (and sounds) nearly identical. Examples: JPEG, WebP (lossy mode), MP3, compressed PDF.
For most everyday uses — sharing photos, emailing documents, reducing storage use — lossy compression at moderate levels is indistinguishable from the original to the naked eye.
What Happens When You Compress a JPEG Image
JPEG compression works by dividing the image into 8x8 pixel blocks and using a mathematical transform (Discrete Cosine Transform, or DCT) to identify which color variations are important and which can be simplified. High-frequency detail (fine texture, sharp edges) is reduced more aggressively than low-frequency information (broad colors, smooth gradients).
At quality level 85 (a common default), the result is typically 60–80% smaller than the original with no visible difference to most people. At quality level 60 (high compression), the result can be 85–95% smaller but may show subtle 'blocky' artifacts in flat-color areas. For email, web, and document use, this is almost always acceptable.
What Happens When You Compress a PNG Image
PNG uses lossless compression by default, which means compressing a PNG does not reduce quality — it only removes encoding inefficiencies. However, there is a second technique that dramatically reduces PNG file size: reducing the color palette. Converting a 24-bit PNG (16 million possible colors) to an 8-bit indexed PNG (256 colors) is technically a lossy step but for screenshots, logos, and graphics with few colors, the result is visually identical while being 50–80% smaller.
What Happens When You Compress a PDF
A PDF file is a container that holds text, vector graphics, images, and metadata. Most large PDFs are large because they contain embedded images — either photos inside the document or scanned pages rendered as images. PDF compression works mainly by re-compressing those embedded images at lower resolution or quality.
For scanned documents (each page is a photo), compression can reduce file size by 60–90%. For text-only PDFs, the file is already small and compression has little effect. For mixed documents, compression targets the image elements only, leaving text perfectly sharp.
What Does Compressing an App Do on Your Phone?
When Android shows 'compressed' in storage settings, it is usually referring to system-level app compression — the OS compresses app code that is not currently running to free RAM or storage. This is different from manually compressing files. You do not control this; Android manages it automatically. What you can control is compressing the photos, documents, and PDFs that your apps produce — that is where tools like MiniFiles help.
Does Compression Change the File Format?
No, in most cases. Compressing a JPEG produces a smaller JPEG. Compressing a PNG produces a smaller PNG. Compressing a PDF produces a smaller PDF. The format and extension stay the same; only the internal data is reduced. Some tools offer format conversion (e.g., converting PNG to WebP or JPEG for better compression ratios), but that is a separate step from compression.
Is Compressed Data Permanent?
For lossy compression (JPEG, compressed PDF): yes. The discarded data cannot be recovered. The original file is not modified — MiniFiles and similar tools create a new compressed file and leave the original intact. But if you delete the original and keep only the compressed version, you cannot un-compress it back to the original quality.
For lossless compression (optimized PNG, ZIP): no data is lost. You can decompress and get exactly the original.
Best practice: keep your originals until you have confirmed the compressed version is acceptable for your purpose.