You have a 14 MB PDF and a form that accepts 1 MB. Compressing it produces something under the limit and entirely unreadable. This guide explains why that happens, what to do instead, and how to avoid the situation entirely next time.
There are two completely different kinds of PDF
This distinction explains almost everything about PDF file sizes, and most advice ignores it.
Text PDFs
Exported from Word, Pages, LaTeX or a web page. Letters are stored as instructions, "draw this character, in this font, at this position", which is extraordinarily compact. A hundred-page text PDF might be 200 KB.
These are already small, and there is very little to compress. If your text PDF is somehow huge, the cause is almost always an embedded image or an unnecessarily embedded font set.
Scanned PDFs
A photograph of each page, wrapped in a PDF. A single page scanned at 600 DPI in colour is roughly 25 million pixels. Ten pages of that is your 14 MB file, and it is where essentially all PDF size problems come from.
How to tell: try selecting a line of text. If you can highlight individual words, it is a text PDF. If your cursor just draws a box, it is a scan.
What compression actually does
Compressing a scanned PDF means re-rendering each page image at a lower resolution and quality until the total fits. That is the only lever available, and it is genuinely destructive:
- Text stops being text. After rasterisation it cannot be selected, searched, or read aloud by a screen reader.
- Links and form fields disappear. Everything becomes a flat picture.
- Small text degrades first. Footnotes, reference numbers and fine print go before body text does.
If your document was already a scan, none of this is a real loss, it was never selectable text to begin with. If it was a proper text PDF, think carefully before rasterising it.
Aim just under the limit, not as small as possible
Most PDF compressors offer "low, medium, high" and let you guess. Guess one way and you are still over the limit; guess the other and you have destroyed a document while leaving most of your allowance unused.
The right approach is to search: rebuild the document repeatedly at different settings, measure the real byte count, and keep the largest version that fits. Asking for 1 MB should give you something close to 1 MB. Our PDF compressor works this way deliberately.
Often the right answer is fewer pages
This is the point most people miss. If a portal accepts 500 KB and you have a 40-page bundle, no compression setting produces a readable result. Forty pages into 500 KB is 12 KB per page, about a postage stamp's worth of detail each.
A tight limit is usually a signal about what is wanted. A form asking for 500 KB is expecting a certificate, not an archive. Extract the pages actually requested and the problem disappears, and extraction is lossless, so those pages stay perfectly sharp.
There is a privacy argument here too. A bank statement submitted as proof of address also discloses every transaction you made that month. Sending one page instead of twelve shares far less.
Greyscale is the easiest win
For scanned text, converting to greyscale often halves the file with no meaningful loss. The colour information in a scan of a black-and-white document is nothing but lighting variation across white paper, it costs bytes and carries no meaning.
Do not use it if the document contains photographs, charts where colour distinguishes series, or a signature in blue ink that needs to be visibly distinct from printed text.
Better: scan properly in the first place
Most oversized PDFs are created by scanner and phone-app defaults that nobody changes.
| Setting | Use | Why |
|---|---|---|
| Resolution | 150–200 DPI | Ample for text. 600 DPI is nine times the data for no visible gain on screen. |
| Colour | Greyscale, or black & white for plain text | Colour triples the data for a document that is black ink on white paper. |
| Format | TIFF is frequently uncompressed and enormous. | |
| Quality preset | "Document", not "Photo" | Photo presets preserve subtle gradation that a document does not have. |
Phone scanning apps default to high-resolution colour, which is why a three-page certificate arrives as 15 MB. Changing one setting once fixes it permanently.
A decision guide
| Situation | Do this |
|---|---|
| Scanned document, moderately over the limit | Compress, greyscale on |
| Many pages, only some needed | Split first, then compress if still needed |
| Several files, one required | Merge, then compress once |
| Photos of documents, PDF required | Images to PDF, then compress |
| Text PDF that must stay searchable | Re-export from the source at a lower image quality instead of rasterising |
| You can rescan | Rescan at 200 DPI greyscale, better than any compression |
Check the result
Open the compressed file and read it, properly, at 100%, including the small print. Compression failures are invisible in a thumbnail and obvious to whoever has to process your application. If the smallest text is no longer legible, you have gone too far, and it is better to discover that now.