Using the block editor, you can add sections of content ("blocks") and reorder them by dragging and dropping.
Click the + in the top left corner to see all the options for different types of blocks, and select a block to add:
"Common Blocks" includes the block types that you'll use most often, including paragraph text, headers, pictures, and file links.
Note that different types of text formatting, like headers, bullet point lists, and quotes, each have their own block type.
Adding an image/picture
Click on the + to add a block, and select the "image" block:
Click on "Upload image" and select the image you want to insert into the post.
Adding a PDF
Click on the + to add a block, and select "File". Click on "Upload" and select the PDF you want to insert into the post. It will create a link for that PDF.
If you hover over the content on the page/post, it will show you what block type it is, like this list block:
You can use the up and down arrows to move the block, or you can click on ::: in the middle to drag and drop.