Start from what you want to do. Each answer says what to expect, including where a tool stops.
Make a PDF smaller
Compress PDF rewrites the file more compactly without touching its content. That works on PDFs made from documents: a 10-page text PDF came out 26% smaller in our test. It does nothing for scans and photos, because their size is in the images: a PDF of photos did not shrink at all.
To tell which kind you have, try to select a line of text. If you can, the page is real text. If it selects as one block or not at all, it is a picture, and only smaller images will help: scan again at 150 dpi in grayscale, or turn the pages into JPGs with PDF to Image (it takes files up to 50 MB) at Standard quality and rebuild the PDF with Image to PDF. The rebuilt pages are pictures on A4 pages with a small margin, so keep the original if you need to select its text.
Put several files into one PDF
Merge PDFs joins whole PDF files, up to 50 MB together, in the order you drag them into. For photos or scans saved as JPG, PNG or WebP, use Image to PDF instead: each image goes on its own A4 page, turned to match the image, and a JPG usually goes in without being compressed again.
To combine images and PDFs, make a PDF of the images first, then merge it with the others.
Sign, fill in or mark up a PDF
Edit PDF lets you type anywhere on a page, draw a signature with a mouse or finger, add shapes and images, and add or delete pages, with up to 50 steps of undo. Your text sits on top of the page: the form’s own fields are not filled in. That is fine for a form you print or email, but a form that checks its fields will not see your text.
For a signature you can use again, draw it once in the Signature Maker and download it as a transparent PNG, then place it with Edit PDF whenever you need it.
Get the text out of a PDF
If you can select the text, PDF to Word rebuilds it as an editable document. A 10-page text PDF with headings and tables converted in 0.9 seconds on our server, with every word and all 10 tables kept as real tables.
A scanned PDF has no text inside it, only pictures of pages, so PDF to Word can only give you those pictures. For a scan, turn the pages into images with PDF to Image at High quality, then read each one with Image to Text, which reads English and Hindi.
Turn PDF pages into pictures
PDF to Image draws every page as a JPG or PNG. Standard is about 144 DPI, enough for screens and chat apps; Maximum is about 288 DPI, which turns an A4 page into about 2381 × 3367 pixels. PNG keeps text and line drawings crisp; JPG makes much smaller files for pages full of photos.
Turn Word or Excel files into PDFs
Word to PDF and Excel to PDF run on our server with LibreOffice and embed the fonts. Calibri, Cambria, Arial and Times New Roman are swapped for fonts with the same letter widths, so text fits where it did in Office. Excel workbooks come out sheet by sheet, with charts, colors and the page setup saved in the file; a 2,000-row sheet made 44 pages in about 3 seconds.
Lock or unlock a PDF
Add PDF Password encrypts the file with AES-256 under one password of at least 6 characters. Without it the PDF will not open, and nobody, including us, can recover a forgotten password, so keep it somewhere safe.
Remove PDF Password takes the password off a PDF you can already open. A PDF that opens without a password but blocks printing or copying unlocks with the password box left empty.