Most PDF advice tells you which buttons to press. That is not where things go wrong. Things go wrong in the small choices around those buttons: the wrong export setting, the wrong order, the "done" that was not actually done. Below are the PDF tasks people redo most often, the specific mistake that causes the redo, and the fix that makes it stick the first time.
Merging: pages flip, rotate, or land out of order
The classic merge failure is not losing pages, it is getting them sideways or shuffled. Two causes. First, scanned pages carry a rotation flag that some viewers honor and some ignore, so a page looks upright in one app and rotated 90 degrees after you merge. Fix it by applying the rotation permanently (a "rotate and save" step) before merging, not just nudging the view.
Second, order. Drag-and-drop merge tools sort by the order you add files, not by filename, so report_1, report_10, report_2 sneaks in. Rename with zero-padding (01, 02, 03) before you import, or reorder inside the tool and confirm the thumbnail strip before exporting. Thirty seconds of checking beats re-merging a 60-page packet.
Compressing: the file shrank but now it is unreadable
People crank compression to the smallest possible size and then wonder why the text looks fuzzy. The trap is that aggressive compression downsamples images, and on scanned PDFs the entire page is an image. Drop the DPI too far and small text turns to mush.
Match the compression to the destination. For email or web viewing, 150 DPI is plenty and keeps text crisp. For print, stay at 300 DPI. If your "PDF" is really a stack of scans, run text recognition first (see OCR below) so the words live as actual text instead of pixels, then compress the images underneath without touching legibility. And always keep the original. Compression is lossy and there is no undo once you have replaced the source.
Converting: layout explodes going to or from Word
Converting PDF to Word and back is where formatting goes to die. The mistake is expecting a pixel-perfect round trip. A PDF is a finished layout; Word is a reflowable document. The converter has to guess where paragraphs, columns, and tables begin.
Two practical fixes. If the PDF came from a scan, you must OCR it before converting or you will get an empty or garbled document, because there is no text to extract. If you only need a few lines, skip the full conversion and copy the text straight from the PDF viewer. When you do convert, expect to fix tables and multi-column layouts by hand; no tool nails those automatically.
OCR: you "made it searchable" but cannot find anything
OCR (optical character recognition) turns picture-of-text into selectable text. The frequent failure is running it and still getting no search hits. Usual reasons: the scan was too low-resolution (aim for 300 DPI), the page was skewed (deskew first), or you picked the wrong language pack, so accented characters and non-English text come out as noise.
Confirm it worked before you rely on it. Try selecting a sentence with your cursor. If you can highlight individual words, OCR took. If your cursor grabs the whole page as one block, it is still just an image and the search box will keep coming up empty.
Signing and splitting: the quiet mistakes
Signing trips people up because a typed name in a text box is not a signature field, and worse, anyone can edit it afterward. Use the tool's actual signature or fill-and-sign feature, then flatten the document so the signature and form entries cannot be altered. If the file is legal or contractual, a proper digital signature backed by a certificate is what actually proves who signed.
Splitting fails when people extract "page 5" and forget some tools count from page 0, or they split by a fixed page count that slices a section in half. Preview the output range, and split on the document's real section breaks rather than guessing at page numbers.
The through-line for all of these: do the one verification step the tool does not force on you. Check the thumbnail order, try to select the text, open the compressed file before deleting the original. That single look is the difference between done and done over.