The quick answer
Start from the original photo, choose the High quality setting, use the same page size as the image or A4, and avoid compressing the pictures before you convert. Then check the PDF at 100 percent zoom. Quality is lost mostly by re-saving, screenshots and heavy compression, not by the PDF itself.
Why quality drops
A screenshot or a photo forwarded through a chat app is already smaller and more compressed than the original. Compressing it again, or choosing a Small file setting, adds blur and blocky edges. A PDF only stores the picture, so it cannot make a poor picture sharper.
Step 1: Use the best source
Use the original file from your phone or camera, not a copy from a messaging app. For documents, shoot straight-on in daylight with the page flat and filling the frame, and keep the text in focus.
Step 2: Choose the settings
Open the image to PDF tool, add your pictures and set the order. Choose High quality for text or details, A4 or Same as image for the page size, and a Small margin. Press Create PDF.
Step 3: Check the result
Open the PDF and zoom to 100 percent. Text should be readable and edges clean. If the file is too big for an upload, switch to Balanced, or compress the pictures to a size such as 500KB before converting.
Many pages in one PDF
Add all the pictures and set the order with the arrows, or follow merge images into one PDF. For documents shot with a phone, see scan image to PDF.
Quick checklist
Original file, sharp and straight photo, High quality, correct page size, check at 100 percent, and only compress if a size limit forces you to. Your pictures never leave your browser.