How to compress a PDF to 100 KB, 200 KB or 1 MB without ruining it

Job portals, scholarship sites, visa applications and bank forms love to say the same thing: "Upload a PDF, maximum 200 KB." Then you look at the scanned certificate on your phone and find it is 6 MB. You try an online compressor, and the result is either still too large or so blurry that the text is hard to read.

Understanding why a PDF is big makes the fix straightforward. This guide explains what is inside a PDF, which levers change the file size, and a simple process to reach a target like 100 KB, 200 KB, 500 KB or 1 MB while keeping the document readable.

What makes a PDF large

A PDF is a container. Inside it can be text, fonts, vector graphics and images, and each of them adds weight in a different way.

Page images. If you scanned a paper document or took photos of it, each page is a picture. This is by far the most common reason for a large PDF. A colour page scanned at 300 DPI is about 2480 x 3508 pixels, which is over 8 million dots, and even compressed it usually takes between one and three megabytes.
Embedded images in a text document. A Word document with a few full-resolution photos can easily reach many megabytes, even if the text itself is tiny.
Embedded fonts. Complete font files can add hundreds of kilobytes, although most tools embed only the characters used.
Extras. Metadata, thumbnails, form data and hidden layers add a little, and are rarely the main problem.

The practical consequence is that a document typed on a computer and exported to PDF is usually small, often well under 200 KB for several pages. A document that came from a scanner or a phone camera is large. If you want to know which kind you have, try selecting text in the file. If you can highlight words, it contains real text. If the whole page highlights as one block, it is a picture.

The levers that reduce size

Four settings account for nearly all the savings. Learn them and you can choose the right trade-off yourself.

Resolution

Resolution is how many pixels represent each inch of the page. It has the largest effect, because file size grows with the square of the resolution. Halving the DPI reduces the pixel count to a quarter.

Resolution	Look	Typical use
300 DPI	Sharp, print quality	Archiving, printing, small print
200 DPI	Very readable on screen	Forms with a size limit of a few MB
150 DPI	Clear text on screen, soft when zoomed	Most online applications
100 DPI	Small text starts to blur	Last resort for a tight limit

Colour or grayscale

A grayscale image holds one value per pixel instead of three. For text documents such as certificates, ID pages, bank statements and letters, grayscale is usually perfectly fine and can cut the size by a third to a half. Keep colour only when the colour carries meaning, such as a stamp or a coloured logo that the reviewer must see.

Image quality (JPG compression)

Scanned pages inside a PDF are usually stored as JPG. Lowering the quality setting makes the picture smaller at the cost of fine detail. Moderate values around 60 to 75 are a good balance for documents. Very low values create halos around letters, which makes text harder to read.

Page count

This sounds obvious, but it is the lever people forget. If the form only needs the first and last page of a long document, send only those two. Every page you remove saves its full share of the size.

A practical method for any target size

Check the target and the current size. A target of 200 KB for a three-page document means roughly 65 KB per page.
Remove pages you do not need. Do this first, since it costs nothing in quality.
Choose grayscale if colour does not matter.
Reduce resolution to 150 DPI and compress. Check the size.
If it is still too large, lower the image quality a step or two, or try 120 DPI. Stop when you are under the limit.
Open the result and read it. Zoom in to 100 percent and check that dates, numbers and names are still clearly legible. The smallest text matters most, for example registration numbers or marks.

As a rough guide, a single A4 page at 150 DPI in grayscale often ends up between 80 and 200 KB, and a colour page at the same resolution between 150 and 400 KB. These figures vary a lot with the content, so treat them as an order of magnitude and not a promise.

Starting with a better scan saves you the trouble

If you have not scanned the document yet, scan it at the target quality the first time. Compressing a poor scan never makes it better.

Use a flat surface and daylight, with no shadows or glare across the paper.
Hold the phone parallel to the page. Angled shots need a perspective correction that blurs the edges.
Use a document scanner mode if your phone has one. It straightens the page, boosts contrast and usually outputs a much smaller file than a normal photo.
Choose black and white or grayscale for text documents.
Crop out the table or floor around the paper. Empty edges still cost bytes.

Text PDFs and the "selectable text" question

Some compressors turn every page into an image, even when the original contained real text. The file gets small, but you lose the ability to search and copy the text, and sometimes the result looks slightly soft. For a typed document that is already small, this is the wrong tool. If a Word-generated PDF is too large, find the embedded image that is causing it and reduce that instead, or export again with a "minimum size" option.

For scanned documents the situation is different. They are pictures from the start, so compressing the pictures is the right approach, and there is no text layer to lose. Some portals even prefer scans to be flattened images.

Combining and splitting documents

Forms often want everything in one PDF, and sometimes one PDF per document. Both can make you exceed a limit.

If you have several photos of certificates that need to be one file, convert and combine them into a single PDF, then compress the combined file once, not each photo separately.
If one combined file is too large for the limit, split it into logical parts, such as identity documents and education certificates, and upload them in separate fields if the form allows.
Keep a copy of the original uncompressed files. The compressed version is for the portal, and the original is for your records.

Common mistakes

Compressing a file that is already compressed. Repeating the process quickly adds blur without saving much more.
Using "print to PDF" on photos. The printer driver may embed the image at a higher resolution than needed.
Chasing the limit exactly. A file at 199.9 KB can fail, since tools count KB in different ways. Aim for roughly 10 percent below.
Forgetting to check readability. The smallest file is worthless if the reviewer cannot read the registration number.

Privacy tip

Your certificates and ID pages are sensitive. Choose a compressor that runs in your browser or on your device, not one that uploads your file to a server you know nothing about. If a tool asks you to create an account to compress a single page, consider another option.

Frequently asked questions

Can I compress a PDF without losing quality?

Only in small ways, such as removing metadata or unused objects. To reduce a file by a large factor you need to trade away some image detail, and the goal is to choose what to lose so the text remains readable.

Why did my PDF get bigger after I merged files?

Merging keeps every page as it was. If one of the files contains a large image, the merged file inherits it. Compress after merging.

What is a reasonable limit for a scanned page?

For clear text, plan on about 100 to 200 KB per page in grayscale at 150 DPI. If a form allows only 100 KB for several pages, you will need to reduce the resolution and the quality further, so check that the final result is still legible.

Why does my phone show a different file size than the portal?

Phones often show sizes in MB with rounding, and some systems use 1000 bytes per KB while others use 1024. Always leave a margin.