Where the megabytes come from
A page of plain text in a PDF is a few kilobytes. A PDF that weighs 8 MB is carrying something else, and it is almost always one of these:
| Cause | Typical size | What helps |
|---|---|---|
| Scanned pages at 300 to 600 dpi | 1 to 3 MB per page | Downsample to 150 dpi and recompress as JPEG; text stays readable, file drops by 70 to 90 percent |
| Photos placed at full camera resolution | 2 to 6 MB each | Downsample to the size they are printed at; a photo shown 8 cm wide never needs 4,000 pixels |
| A logo or letterhead embedded once per page instead of once per file | 50 to 500 KB per page | Deduplicate identical image objects |
| Fully embedded fonts, unused pages, revision history | 0.2 to 2 MB total | Subset the fonts, remove unreferenced objects, save as a clean copy |
| Uncompressed content streams from older generators | varies | Apply stream compression; lossless, no visible change |
The practical consequence: a PDF that is mostly text will hardly shrink no matter what you do, and a scanned PDF will shrink a lot with the right image settings. Knowing which one you have saves the guessing.
Two methods, one important difference
Every PDF compressor uses one of two approaches, and the difference decides whether the result is still a document or just a stack of pictures.
- Structure-preserving compression opens the file, recompresses the images inside it, compresses the content streams, subsets fonts and removes unused objects. The page structure stays as it was: text remains selectable and searchable, links work, form fields and bookmarks survive, and screen readers can still read the tags.
- Rasterisation renders each page to an image and builds a new PDF from those images. It reaches very small sizes even for complex pages, but the result has no text layer, no links, no form fields and no accessibility structure. A signed contract turned into pictures is no longer the document that was signed.
Some free online tools rasterise by default because it is the easy way to guarantee a small file. If a portal later rejects the upload because the PDF is "not text-searchable", that is why. Use rasterisation only as a last resort and only when you know the receiver does not need the text layer.
Compressing to an exact limit
A quality slider is the wrong control for an upload limit, because it sets an encoder parameter and hopes. The right control is the limit itself:
- Enter the maximum size the portal accepts, in KB or MB.
- Start with structure-preserving compression at moderate image settings and measure the real output size.
- If the file is still too large, lower the image resolution and quality in steps and measure again. Stop at the largest result that is under the limit.
- Only if the lowest acceptable image settings still miss the target, decide about rasterisation. That decision should be yours, not the tool's default.
- Verify the final file: it opens, has the same number of pages, and its byte size is at or below the limit. A result that is "about 2 MB" can still be 2,097,153 bytes and fail.
The Target Size PDF Compressor works exactly this way. It takes up to 20 PDFs of up to 50 MB each, clones the document catalog so forms, links, tags and bookmarks survive, compresses in steps towards the target you entered, and reopens each result to verify page count and bytes. The image-only fallback is off by default and clearly reported when used. Password-protected and digitally signed PDFs are rejected before a job is counted, because rewriting them would break the protection or the signature. Three files a day are free without an account, and the ZIP includes a report with the original bytes, the result bytes, the method used and a hash of every file.
Common limits and what usually fits
| Limit | Where it appears | What fits without rasterising |
|---|---|---|
| 500 KB to 1 MB | Application forms, some government portals | Text documents with a logo, 2 to 5 scanned pages at 150 dpi |
| 2 MB | Job applications, university admissions, insurance claims | 10 to 20 scanned pages at 150 dpi, a CV with one photo, most contracts |
| 5 MB | Email gateways, tax portals | 30 to 60 scanned pages, a brochure with a dozen photos |
| 10 MB | Tender platforms, court filing systems | Almost everything except photo-heavy reports and high-resolution drawings |
If your file is far above the line for its limit, the fastest fix is usually upstream: rescan at 150 dpi in greyscale, or export from the source application with "reduce image resolution" turned on. Compression afterwards can only work with what the file contains.
Things that do not help
- Zipping a PDF. The images inside are already compressed; a ZIP saves a few percent and most portals do not accept it anyway.
- Printing to PDF again. Depending on the driver this either changes nothing or silently rasterises the pages.
- Splitting into several files when the portal expects one document. It works for email, not for a form that has one upload field.
- Lowering quality below what a reader can check. A scanned ID or a signature that has turned to mush will be sent back, and then you upload twice.
PDF compression FAQ
Why is my PDF so large?
Almost always because of images: scanned pages, photos at full camera resolution, or a logo embedded separately on every page. Fonts and leftover data add to it, but rarely by megabytes.
Does compressing a PDF make the text unselectable?
Not if the tool recompresses the images and streams and leaves the page structure alone. Text becomes unselectable only when every page is turned into a picture.
How small can a PDF get without losing quality?
A scanned document usually compresses to a quarter or less of its size at 150 dpi without visible loss on screen. A PDF that is mostly text barely shrinks, because there is nothing large to remove.
Can I compress a PDF to exactly 2 MB?
Yes, by starting from the limit: compress in steps, measure the real bytes and keep the largest result under the target. If the target cannot be reached without flattening pages, the tool should say so rather than hand back a file that fails the upload.
Enter the limit, get a verified file
Up to 20 PDFs, target in KB or MB, text kept selectable unless you allow otherwise. Final bytes and page count are checked before download.
Open the Target Size PDF Compressor