As an alternative to Photoshop, GIMP (GNU (GNU‘s Not Unix Image Manipulation Program) is a great and free photo editor that can support large file formats like BMP (bitmap), GIF (Graphics Interchange Format), JPEG (Joint Photographic Experts Group), MNG (Multiple-image Network Graphics), PCX (PiCture eXchange), PDF (Portable Document Format), PNG (Portable Network Graphics), PS, PSD (Adobe PhotoShop Document), SVG (Scalable Vector Graphics), TIFF (Tagged Image File Format), TGA (Truevision TGA/TARGA), XPM (X PixMap)… and for RAW image format, you need to install darktable (as an alternative to Lightroom). Darktable is a non-destructive editor, so if you change anything on your raw files everything is stored in a database and the original image is untouched. After making all the changes you need to export (CTRL+E) and convert the raw file into a jpeg and then you can edit it with GIMP.
For camera support and manuals check https://www.darktable.org/resources/.
GIMP User manual can be found at https://www.gimp.org/docs/ and some tutorials at https://www.gimp.org/tutorials/.
You can find the latest version of GIMP at https://www.gimp.org/ on the Download menu.