How PDF compress works, including the quality you give up
Compress redraws the page
This compress task is for a PDF that is too large to send, usually because the pages are photographs or scans. It redraws each page as a JPEG at the quality the slider starts from. The sample is a one-page photo-style scan. There is no box where you type a target file size. You choose a quality, export, and look at the result. Text that was sharp vector type can become soft, and a page that was mostly text can look worse than a page that was already a photo. If the PDF is already small, or you need every letter to stay selectable and sharp, do not compress it.
Merge does not shrink files. Split does not shrink files. Only this redraw does, and the shrink is the quality you spent. The compress guide explains how to decide before you overwrite your only good copy. Image files that are not PDFs belong in image compress, which exports JPEG, WebP, or PNG from a picture rather than from a PDF page. Do not expect the two tools to produce the same kind of result.
Check the export before you send it
Open the compressed PDF and read the smallest text you care about. Look at a photo for blocks and muddy color. Compare the file size with the original only after the page still does the job. If it does not, raise the quality and export again from the original, not from the already compressed file. Compressing a compressed file stacks the damage. The work stays in this browser. NexTool does not upload the scan to compress it, and it does not keep either version. Password-protected files still need to be unlocked first. A file that fails for lack of memory should be handled on a machine with more free RAM, or split into smaller pieces before you compress.
A useful test is to print one page to paper or to zoom to 200 percent on screen. If a signature, a stamp, or a table border breaks into squares, the quality is too low for that document even if the file is now small enough to email. Raise the slider and try again from the original. Scans of handwriting need more quality than scans of a clean printed form. Color photos need more than a grayscale receipt. There is no single number that is “without quality loss.” Loss is the method. The honest goal is the smallest file that still lets the reader do the job, not the smallest file the slider can make.
When to leave the file alone
Skip compress when the PDF is already a few hundred kilobytes, when the recipient must copy a sentence out of it, or when the pages are line drawings that must stay crisp. Compressing those pages spends sharpness to save space you did not need. Use it when a photo scan will not pass a mail limit and a slightly softer page is acceptable. Export from the original each time you try a higher slider. Do not compress the already compressed file. Compare the result at a large zoom, then send that file and keep the scan you started with. The redraw happens in this browser. Nothing is uploaded to make the file smaller.