Which tool for which task?
Use merge PDF to send several documents as a single file, and split PDF to extract only the pages you need from a long one. When an email provider or an online application form has a size limit, compress PDF re-encodes the images inside the pages to bring the file size down. To turn phone photos of documents into one PDF, use JPG to PDF; to put a PDF page into a presentation or social post, use PDF to JPG.
Privacy: your files stay on your device
Many online PDF services upload your file to their servers and process it there. These tools use open-source libraries that run directly in your browser, so sensitive documents such as ID copies, contracts or bank statements never leave your device. Processing speed therefore depends on your device rather than your internet connection, and there is no artificial limit on the number of files. Very large documents may take a few seconds on older phones.
Tips for better results
Put the files in the order you want before merging and check the preview. Compressing a scanned document usually saves a lot of space, while a PDF that contains only text barely shrinks, because there are hardly any images to compress. Password-protected PDFs need to be unlocked before they can be processed. When creating a PDF from photos, shoot the document flat, from directly above and in good light to keep it readable, then check the page order in the result.