The answer is decided by the file, not by preference. docx is for Word documents someone will keep editing. pdf is for a fixed document you need to read, split, fill or produce as a final artefact. Most document work ends up needing both.
The docx skill lets Claude create and edit real Word documents, including headings, tables, page numbers and tracked changes, instead of returning text that you then have to format yourself. Install it when the deliverable itself is a Word file. If you only need the words, plain Markdown is faster and cheaper.
Best for
Reports, letters, contracts and templates where the recipient expects a .docx file with working formatting.
Skip if
Your output ends up in a web page, a wiki or a code repository, where a Word file is a step backwards.
Which one to install
Choose docx when the output has to open in Word and survive being edited: reports with headings and a table of contents, letters on a template, anything a colleague will change after you hand it over. A PDF cannot carry that structure forward.
The pdf skill gives Claude real PDF operations: extracting text and tables, merging and splitting files, rotating pages, filling forms, adding watermarks and running OCR on scans. Install it whenever PDFs are an input or an output of the work, rather than something you only need summarised.
Best for
Invoice and report pipelines, scanned archives, and any task that ends with a single assembled PDF.
Skip if
You only need the gist of one short document, which the model can already read without extra tooling.
Which one to install
Choose pdf when the document is already fixed, or has to be. Reading and extracting from one, merging or splitting, filling a form, or delivering something that must look the same everywhere. This is also the skill for scanned material that needs text recovered.
Installing both is normal and they do not fight, because the trigger for each is a file type rather than an activity. The pairing that actually needs a decision is the one between producing a document and producing a spreadsheet.