Which mode you want
A single break is usually a mid-paragraph wrap you want gone. A double break, the blank line between paragraphs, marks a real boundary worth keeping. That distinction decides whether you get readable prose or one flat block.
Join into one line: every break becomes a space. For a wrapped paragraph that needs to be running text again.
Join with nothing: the two halves meet directly. Use it on a URL or a word split across two lines.
Keep paragraphs: single breaks go, double breaks stay. PDF and email text usually needs this, since the paragraph spacing is the only structure worth saving.
Replace with a comma or semicolon: each line turns into a list item. The quiet workhorse of the page. Copy a column of names or IDs, swap the breaks for a comma, and you have an inline list ready for a search box or a SQL IN clause.
Remove all line breaks: everything collapses to one line, structure included.
Two toggles help with a messy source. "Remove blank lines" drops empty rows, and "collapse extra spaces" squeezes runs of spaces down to one. Both combine with any mode.
Why PDF text breaks on every line
A PDF records where every letter sits on the page, not that a run of words is one paragraph. The visual wraps are stored as explicit breaks so the words fit the column, and copying brings them along at the end of every line.
Justified PDFs add hyphenation on top. A word split as "visual-" and "ization" rejoins as "visual- ization", so check dense justified text after flattening it.
CR, LF and CRLF
Three invisible characters do all of this work. LF (\n, U+000A) ends lines on Linux and modern macOS. CR (\r, U+000D) was the ending on classic Mac OS. CRLF uses both and is still how Windows ends every line.
Anything that strips only \n leaves CR-only and CRLF text half-cleaned. That is behind the Excel complaint that Ctrl+J finds no line breaks: it matches CHAR(10), the LF, and misses the CHAR(13) carriage returns legacy imports leave in a cell. All three read the same way here.
On a Mac the same cleaners sit in Klipto's paste preview, so PDF text arrives joined in Slack, Notion or an email with no browser tab in the middle.