Why PDF compression can increase file size.

A PDF compressor is not guaranteed to make every file smaller. Some PDFs are already optimized, some contain efficient text objects, and some include image data that cannot be reduced further without making the document unreadable. Understanding the file type helps you choose the right next step.

A PDF is not one single format inside

Two files can both end in .pdf and behave completely differently. One may contain selectable text, embedded fonts, vector shapes, and small images. Another may be a collection of full-page camera scans. The compressor has more room to reduce the second type because it can scale and recompress images.

If a text-based PDF is already clean, rewriting the file can add new structure, metadata, or image previews. In that case, a processed file can become larger even though the tool did not fail.

Start with page count and source quality

When a portal asks for one certificate, split that page before compression. Compressing pages that will not be uploaded wastes quality and rarely helps. A one-page source gives the tool a better chance to stay under the required limit.

For scanned files, source quality matters. A document photographed on a desk with shadows, borders, and background objects contains extra pixels that must be stored. Cropping and rescanning with even light can reduce file size more cleanly than repeated compression.

Do not keep the larger result

Always compare the output size with the original. If the result is larger, keep the original and try a different method: split pages, export from the original app with smaller images, convert only the required page to PNG, or reduce scan resolution before making the PDF.

Repeatedly compressing the same larger result usually makes quality worse while saving little. Return to the original file whenever you change strategy.

When a strict KB target is unrealistic

Some limits are too small for a readable multi-page scan. A 100 KB target can work for a simple one-page certificate, but a document packet with stamps, signatures, photos, and dense text may need more space. The file should remain readable before it is submitted.

If the portal allows separate uploads, split the document into the fields requested by the form. If the portal requires one file, use the least aggressive compression that passes the limit and inspect every important detail before upload.

Common questions

Does a larger compressed PDF mean the tool is broken?

Not always. It often means the original PDF was already optimized or the compressor had to rewrite the file with structure that increased the final size.

What should I do when compression makes the PDF larger?

Use the original file, reduce page count, crop scans, or export from the source document with smaller images instead of recompressing the larger output.

Can every PDF be reduced under 100 KB?

No. Page count, scan detail, images, fonts, and required readability decide whether a strict size target is practical.

Continue with the right tool

After checking the rule, choose the tool that changes only what needs to change. That keeps the final file easier to inspect and reduces the chance of creating a smaller file that the receiving website still rejects.