The bit you Googled because nobody explained it properly.

Short answers first. The useful detail second. Then the tool, if there is one.

Why is my PDF too big to upload?

Because the upload box has a size limit, and PDFs are very good at quietly collecting heavy images, scans and embedded assets.

Usually because the site resized or recompressed it, or because the original did not have enough pixels for the size it is being displayed at.

Sometimes. The useful question is not whether it looks readable to you, but whether the contrast is strong enough for the text size you are using.

Because the logo is covering some of the code. QR codes can recover from missing information, but only up to a point.

It means compare two fingerprints. If the values match, the content you checked is the same content that produced the reference checksum.

Because some sites are not asking for a generally ‘strong’ password. They are asking for an exact recipe.

There are lots of possible reasons, but one of the easiest to miss is that the page itself is quietly telling search engines not to index it.

Sometimes the title tag is not what you thought your template was outputting. Other times Google rewrites the title it chooses to display.

Usually because old rules have accumulated. The browser hides the detours, so the chain can sit there unnoticed for years.