How to reduce file size
Photos, video, PDFs and documents each shrink for different reasons. Here is what actually works for each, and where the sensible floor is.
Almost everyone who searches this has hit a limit: an upload form, an email attachment cap, a portal that rejects anything over 2 MB. The answer depends entirely on what kind of file it is, because the reasons a photo is large have nothing to do with the reasons a PDF is.
Images
Most photos are large because of resolution and quality settings, in that order.
A modern phone photo is around twelve megapixels. Displayed on a screen, or attached to a form, almost none of that detail is visible — the receiving website will scale it down to a fraction of its size before anyone sees it. Reducing the dimensions before you upload loses nothing you would have seen.
After that comes compression quality. JPEG lets you trade detail for size on a sliding scale, and the useful range is wider than people expect: dropping from maximum quality to around 70% typically halves the file with no visible difference on screen.
The practical approach is to name the size you need and let a tool search for it, rather than guessing at a quality number and re-checking.
The floor: below roughly 30 KB, a photograph stops looking like a photograph. If your target is under that, you need a smaller crop rather than harder compression.
Video
Video size is bitrate multiplied by duration, and that formula is the whole story.
Because duration is a multiplier, the single most effective thing you can do is trim. Cutting thirty seconds off a two-minute clip removes a quarter of the file before any re-encoding happens, and loses nothing in the part you keep.
After that, resolution: 4K to 1080p cuts the pixel count by three quarters. For anything watched on a phone or in a chat window, the difference is invisible.
The floor: below about 500 kbps at 1080p, motion turns to mush. If the target demands less than that, reduce the resolution rather than the bitrate.
PDFs
PDFs are almost never large because of the text. They are large because of the images inside them — usually scans, usually at print resolution, usually embedded uncompressed.
A scanned document at 600 dpi is enormous and no more readable on screen than the same scan at 150 dpi. Re-scanning or downsampling the embedded images typically cuts the file by 80% or more.
The second cause is embedded fonts. A PDF can embed a complete font family when it only uses a dozen characters of it. Subsetting keeps just the glyphs actually used.
Office documents
A Word or PowerPoint file is a zip archive. Almost all of its size is pasted images — often screenshots pasted at full resolution and then dragged small on the slide, which keeps every original pixel.
Both Word and PowerPoint have a built-in “compress pictures” function that fixes exactly this, and it is usually the entire answer.
What does not work
Renaming the extension. Changing .png to .jpg changes nothing about the bytes.
The file is still a PNG and now lies about what it is.
Zipping an already-compressed file. JPEG, PNG, MP4 and PDF are already compressed. Zip finds no redundancy left and saves a percent or two, sometimes making the file slightly larger.
Repeated compression. Compressing an already-compressed JPEG degrades it again without saving much, because the first pass already removed what the format could cheaply discard. Always go back to the original.