What a PDF can tell people about you
People often send a PDF without thinking about what it says about its maker. A PDF exported from a word processor or a design program usually carries an author name, often copied from the user account or the program's settings, the name and version of the program, and the dates it was created and last changed. Some files carry a company name or other entries a program added, and some carry an edit history in the XMP packet: when it was saved and by which program.
How much a file reveals depends on how it was made. A web page printed to PDF usually carries the page title and the name of the browser's PDF library, and little else. A screenshot or a photo saved as a PDF often carries only the name of the system or app that saved it. A document from a scanner app often names the app and sometimes the device. The report on this page shows what your file contains, so you do not have to guess.
Design programs leave more: private notes in the file (PieceInfo), their own metadata on pages and pictures, and small page thumbnails, which can show an older look of a page. And a file saved by adding to the end of the previous version can still hold the earlier title, author and metadata as objects nothing points to. They do not appear in a viewer's properties, but they are in the file.
What removing takes out, and what it cannot hide
Removing here means the data is gone from the file. The Info dictionary is dropped and deleted, the XMP stream is deleted, page and picture metadata, PieceInfo and thumbnails are deleted, and every object nothing points to is dropped, including old copies of the above. The new file is then read again and each of these is listed as a line: none left, or how many are.
Some things are not metadata, so they stay. The pages show their content, and a name printed on a page is still printed on it. Comments keep their authors' names; delete the comments in a PDF viewer to remove them. Fonts can carry a name, and a font made for a company can carry its name and a copyright text inside the font file. Pictures inside a PDF are stored as they were, and a JPEG photo can still carry its own camera model and GPS position. This tool does not rewrite pictures, so strip a photo's data before you put it in a PDF if that matters. A digital signature names its signer, and removing metadata breaks the signature anyway.
The file's own name is not inside the file but it can say as much as any entry, so check it before you send the file. The file name box lets you change it.
Attachments and JavaScript
Two other kinds of hidden content can be removed with a separate switch. Attached files are other files carried inside the PDF, listed in the document or pinned to a page with a paperclip, and whoever you send the PDF to can open them. JavaScript is code a viewer may run when the file opens, when a page is shown, when a field changes or when a link is clicked. The report lists the names and sizes of attached files and counts the scripts.
The switch is off by default because scripts can be part of how a document works. Fillable forms use them to add up totals, check dates and format numbers, and without them the form stays fillable but stops calculating. XFA is a different kind of form that most viewers cannot fill. The page tells you if the file has one and does not touch it.
Checking the result yourself
The lines under the download come from reading the new file again with the same library that wrote it, so they say what is in the bytes, not what the code meant to write. They show the page count and whether every page's content matches the original, then the Info entries, the XMP packet, the metadata streams, the PieceInfo entries, the thumbnails and the unreferenced objects. If one is not as it should be, the file is not offered for download.
For a check of your own, open the new file in a PDF viewer and look at its document properties: author, title, creator and producer should be empty and the dates blank. A second program reads the file with different code, so it gives an independent answer.