It is a common moment of confusion: you run the same PDF through two different free compression tools, and one gives you a 400 KB file while the other gives you 650 KB, or one photo compressor produces a noticeably sharper result than another at what claims to be a similar setting. Neither tool is necessarily broken or lying to you — they are simply making different decisions about the same underlying trade-off.
There is no single, objectively correct compression setting
Compressing an image or a PDF's embedded images always involves a trade-off between file size and visual quality, and where exactly to draw that line is a judgement call, not a fixed, universal answer. One tool's “medium” setting might prioritise keeping more visual detail at the cost of a larger file; another tool's “medium” might prioritise a smaller file at the cost of slightly more visible softening. Both are legitimate choices — they are just different choices, made by different engineering teams with different priorities.
What actually varies between tools
- The underlying compression algorithm and its specific implementation, which can differ meaningfully even between tools using the same general approach (like JPEG compression), since there is room for different technical choices within that approach.
- How aggressively each named setting compresses — one tool's “light” can be closer to another tool's “medium.” The labels are not standardised across different tools.
- Whether the tool does anything beyond simple compression, such as also resizing dimensions slightly, adjusting colour data, or stripping metadata, all of which affect the final size independently of the compression setting itself.
Why this is not a sign that one tool is simply better across the board
It is tempting to conclude that whichever tool produced the smaller file “won,” but a smaller file at the cost of noticeably worse visual quality is not actually a better result if quality matters for your purpose — and the reverse is true if you specifically need the smallest possible file and can tolerate more quality loss. The right tool for a given task depends on what you are optimising for, not on which one produces the smaller number by default.
How to actually judge which result is better for your specific need
- Decide what you actually need first — a specific size limit to hit, or the best quality within a rough size range.
- Compare the results at the size and context they will actually be used in, not just by looking at the numbers.
- If size is the hard constraint, the tool that reliably hits your target with the best remaining quality is the better choice for that task, regardless of what its settings are labelled.
- If quality is the priority and size is flexible, compare the visual result directly rather than assuming a larger file is automatically better.
Why running the same file through the same tool twice can also give different results
Beyond differences between tools, running the exact same file through the same tool at the same nominal setting more than once can occasionally produce very slightly different results too, depending on how the tool's processing pipeline is implemented. This is normal and rarely meaningful in practice; small variations of this kind do not indicate anything is wrong.
Applying this thinking when a specific size target genuinely matters
If you have a hard target — a portal's stated 2 MB limit, for instance — the practical approach is not to assume any particular tool's specific setting will hit it, but to compress, check the actual result against the target, and adjust the setting up or down accordingly, exactly as covered throughout the size-target guides on this blog. Treat every tool's settings as a starting point to check and adjust, not a guaranteed number.
Why this does not mean you cannot trust any of these tools
Different results between tools reflect different reasonable engineering decisions, not necessarily poor quality or untrustworthy behaviour from any of them. The variation is a normal feature of how compression works, not evidence that something is being done incorrectly. What is worth checking, separately from this, is whether the tool is trustworthy with your file in the first place — covered in our guide on telling a good online tool from a risky one.
Sticking with one tool once you find a result you trust
Once you have found a tool whose results consistently meet your needs at a setting you understand, there is little value in repeatedly shopping around for a marginally different result on future files. Consistency has its own benefit here: knowing roughly what a given setting on your usual tool will produce, from experience, lets you skip the trial-and-error step that comparing multiple unfamiliar tools would otherwise require every time.
Frequently asked questions
Should I always use the tool that gives the smallest file?
Only if size is your primary concern and you have checked the quality is still acceptable for your purpose; the smallest file is not automatically the best result.
Is it worth trying more than one tool for an important file?
For something where getting the best possible balance matters, comparing two tools' results directly is a reasonable extra step, though for routine, everyday use this is rarely necessary.
Does a more expensive or paid tool always give better results than a free one?
Not necessarily — price does not directly correlate with compression quality; a well-built free tool can outperform a poorly optimised paid one, and vice versa.
Why does one tool's "high quality" setting produce a bigger file than another tool's equivalent?
Because the labels are not standardised across tools; each tool defines its own settings independently, so the same word can mean a different actual compression strength.
Is there an objective way to measure which result is actually better?
For most everyday purposes, your own visual judgement at the size the image or document will actually be used is the most practical measure, rather than any single technical metric.
Does this variation apply to file conversion too, not just compression?
Yes, to a lesser degree — different tools can produce slightly different results even for a straightforward format conversion, depending on their specific implementation choices.
Is this tool's compression trustworthy?
Yes. The PDF Compressor and Image Compressor here are free, require no sign-up, and files are removed from the server automatically about an hour after processing.