It's hard to know whether a file is "too big" without a reference point. A 3MB file might be completely normal for one kind of document and wildly bloated for another. Here's a rough, practical sense of what's typical, so you can tell at a glance whether something needs compressing or is already fine.
Photo ID / passport-style photo
Typical requirement: under 50-300KB, depending on the specific form. These are small, tightly cropped, single-subject images, so there's no legitimate reason for one to be several megabytes — if yours is, it's almost certainly still at full camera resolution and needs resizing, not just compressing.
A single scanned page (text document)
Typical range: 100-500KB per page at a resolution that keeps text crisp. A scanner or scanning app set to a high DPI in color mode can easily produce 2-5MB for a single page of mostly white space with black text — grayscale mode and a lower DPI (150 is usually plenty for text) brings that down dramatically with no loss of legibility.
A multi-page PDF (contract, resume, report)
Typical range: 200KB-2MB total for a text-based document of a handful of pages, even with a logo or a couple of small images. If a 5-page PDF is pushing 10MB+, the cause is almost always embedded images at far higher resolution than the page needs — see how that resolution mismatch happens.
A photo meant for a website or social post
Typical range: 100-500KB, saved at whatever pixel dimensions the platform actually displays (rarely more than 2,000px wide even for a "large" image online). Full camera-resolution photos, often 3-8MB, are massive overkill for a screen that will never render more than a few hundred to a couple thousand pixels across.
A quick gut check
If a file is meaningfully bigger than these ranges, the almost-always cause is one of two things: it's still sitting at the original capture resolution (camera photo or high-DPI scan) rather than a resolution appropriate to how it'll actually be viewed, or it's an uncompressed format (like PNG) being used for something that would compress far better as JPEG — see the format comparison guide for when each one actually makes sense.
Fixing it without guesswork
Rather than trying quality percentages until something feels small enough, it's more reliable to work backward from an actual target: type in the KB number a form requires (or a sensible number from the ranges above) and let a tool step compression down until it lands there. That's the whole idea behind a compress-to-target-size tool — you set the destination, not the dial.