Open one PDF, pick a job and download the result. The original file is never changed.
What each option does
Extract images finds the pictures stored inside the PDF, such as photos and logos, and saves each one as a PNG at the size it was stored. A scanned document usually holds one picture per page. Drawings, charts and text made of lines and shapes are not pictures, so they are not listed.
Grayscale redraws every page in shades of gray at the resolution you choose, for printing without colour ink. Page size puts every page on A4, US Letter, Legal, A3, A5 or your own size, scaling the content to fit. Details shows the hidden title, author and dates and lets you change or remove them. Compress to a size finds the sharpest version of the PDF that fits under the limit you pick.
Which options keep text selectable
Changing the page size and editing the details keep your text, fonts and drawings exactly as they are, so the text stays sharp and searchable. Links and form fields are not carried over when the page size changes. Grayscale and compress to a size turn each page into a picture, which is why text can no longer be selected afterwards. Keep your original if you might need to copy text from it later.
Tips for reaching a size limit
Text pages compress far better than photos. If a target cannot be reached, you get the smallest version that could be made and an explanation. Removing pages you do not need, or splitting a long document into parts, is the most reliable way to get under a strict limit such as 100 KB. A PDF that is already under the target is left as it is.
Privacy
Your PDF is opened and processed by your browser. Nothing is uploaded, and the file is gone when you close the page. A PDF locked with a password needs to be unlocked first, for example with Unlock PDF.